The 3rd Massachusetts Artillery Battery C was also known as Martin’s Battery. During the battle of Gettysburg, it served as a member of Martin’s Brigade in the Fifth Corps, Army of the Potomac. The battery was commanded at the Battle of Gettysburg by Lieutenant Aaron Francis Walcott (1836 - December 11, 1907), previously, a bookkeeper from Boston. Walcott enlisted April 25, 1861 and mustered in on July 31, 1861. He mustered out on September 16, 1864 but then re-enlisted and drilled the battery in Boston. In 1869 and 1870 he was a merchant tailor, then he went into the real estate business, staying in Boston. IN 1872, he moved to Chicago, continuing real estate an also the storage business. In 1900 he returned to Massachussets and lived in Melrose. While in Chicago he served as a deputy sheriff of Cook County for 8 years. Under Walcott's command the unit brought 124 men to the Gettysburg Battlefield operating 6 Napoleons. There was a total of 6 casualties.
The primary monument for the 3rd Massachusetts Battery Monument can be found 
HERE. That monument is located is south of Gettysburg on Wheatfield Road near the Weikert Farm Lane.
The 3rd Massachusetts Battery - US Battery Marker is located on Wright Avenue, on the right or northeast side of the road if traveling southwest along the Avenue. This is the first monument on Wright Avenue. The tablet is 568 feet from the intersection of Taneytown Road (Route 134) and Wright/Howe Avenues. As a point of reference, at the intersection, a left turn is Wright Avenue and a right turn is Howe Avenue, even though they are the same road, if traveling north. The tablet is flanked by two 12-pounder Napoleon smoothbore guns, both pointing south southwest presumably in the direction of the once advancing enemy. If facing the tablet, the gun on the left is registry #86 from the Alger foundry and the gun to the right is registry #178 from the Revere foundry. 
Parking is on Wright Avenue. Stay off the grass or anything green or you will be ticketed by the Park Police. I visited this site on Monday, July 1, 2013 on the 150th anniversary of the Battle of Gettysburg @ 1:26 PM, EDT & @ an altitude of 516 feet, ASL. As always, I used my trusty and oft abused Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
These cast iron monuments were first built in 1900 and concluded in 1906 according to the NRHP nomination form. Some sites have the monuments being erected in 1910 or later. All of this tablet nonsense is very confusing and disorganized. 1910 seems to be the most reasonable, extremely conservative date so we will go with that. (These tablets are mentioned as early as 1902 in the yearly Commission reports.) This tablet is unique in that I have found no other ones that look like it which leads me to believe that this may be a replacement tablet, a fact which was later confirmed by one of my trusty sources. At any rate, it is amazing with all the work and effort exerted to make the Gettysburg National Military Park a reality, no one took the time to keep accurate records and correct installation dates. The work was done under the direction of the Gettysburg National Park Commission (established by the United States Department of War), after they took over the administration of the park from the Gettysburg Battlefield Memorial Association (whose funds had expired) on March 3, 1893, and whose stewardship was then transferred to the National Park Service in 1933. SOURCE
Draw the Sword, using descriptive help from the NPS site (and me), offers the following description: tandard iron War Department battery position tablet. Painted inscription tablet, 3'8"x3'. Mounted on fluted post, 4'4" high. Marks position of Walcott’s Massachusetts US Battery on July 3, 1863.
These battery tablets were cast by Calvin Gilbert. Gilbert joined the 87th Pennsylvania Volunteers when the "Rebellion" broke out. Since he was musically inclined, he was made a member of the regimental band. In early 1863, he was promoted to the rank of Captain and sent to Washington D.C. for commissary duty. He served in that capacity until the war ended in 1865, and at war’s end earned the brevet rank of Major. 
After returning home to Gettysburg, Major Gilbert moved his family to the town of Chambersburg, where he served as the Superintendent of Schools. He also opened an iron foundry there in 1868. When Calvin learned, nearly thirty years later, in 1894, that the Gettysburg Foundry was available, he purchased it with a business partner & moved his family back to Gettysburg. For the next twenty years, the Calvin Gilbert Foundry created ironworks that are still seen today on many historic forts and national battlefields (this tablet), from upstate New York to Florida to Lookout Mountain. Gilbert lived to be 100 years old and attended the Last Reunion of the Blue and Gray for the 75th anniversary of the Battle of Gettysburg when he was 99. He was a most remarkable man and lived an amazing life.
There is an excellent excerpt in the American Guide Series about Gilbert attending the 75th reunion anniversary.
Look carefully on the reverse side of one of these tablets and his name might be there; I found a tablet or two like this. The inscription on the monument reads:
Army of the Potomac
Fifth Corps Artillery Reserve
Battery C
Massachusetts Light Artillery
Six 12 pounders
Lieut. A.F. Walcott commanding
July 3 At 3 a.m. moved to and occupied this position until the close of the battle.
There are hundreds of non-sculpted monumentation for the various Confederate States & Union units which fought at Gettysburg. There are about 10 different variations of tablets, markers and monoliths by my reckoning. Six designs represent brigade (2), division (2), corps (2), & army headquarters (2), each different in a subtle way so as to distinguish between armies. Battalion/battery/advance position markers and regimental monuments for United States Regulars & Confederate have their own distinct design (2), bringing the total to ten designs at the battlefield. The bronze tablets (the brigade monuments) were created by Albert Russell & Sons Co. of Newburyport, Massachusetts. The granite bases which accompany the various tablets were created by the Van Amringe Company out of Boston, Massachusetts. The cast iron tablets were of course manufactured by the aforementioned Calvin Gilbert. The 1910 Gettysburg Commission report lists the awarded contracts to these companies (not Gilbert) for the tablets. All of these tablets were designed by architect Colonel Emmor Bradley Cope (July 23, 1834 - May 28, 1927). He designed pretty much every tablet for both the Union and Confederate armies, each one distinct, with several different varieties. He is buried in Evergreen Cemetery in Gettysburg, as well he should be as this park and most of the things seen here today were designed by him and as such, are his legacy. These monuments/tablets were erected just after the turn of the century during the first and beginning of the second decade of the 20th century. Each one has since been preserved or restored at least twice since the turn of the 21st century. The plaques and tablets were erected by the Gettysburg Park Commission (established by the War Department). The Gettysburg Park Commission is also referred to as the Gettysburg National Military Park Commission or the Gettysburg National Park Commission, to clear up any confusions, especially my own.
On October 1, 1898, the Gettysburg National Park Commission in a letter to the Secretary of War set gave recommendations for continuing the task of organizing and progressing the work of the Gettysburg National Military Park. Every November they wrote their annual report which outlined the work of the GNPC for that year. The following is an excerpt from that report relevant to this waymark. The link at the end of the paragraph will take you to the entire report. The method of marking the positions of troops on this field, as approved by the War Department, is to place the principal tablet or monument of each command at the position occupied by the command in the main line of battle, and to mark the several important positions subsequently reached by each command in the course of the battle by subordinate and ancillary tablets, with appropriate brief inscriptions giving interesting details and occurrences and noting the day and hour as nearly as possible. SOURCE
Most of the Commission reports have been digitized and can be found HERE. The initial 1893 report up to and including 1921 can be found here. 1922 to 1926 are missing and have yet to be discovered. The years 1927 to 1933 were discovered in 1996 and are also included. To my knowledge this is the most comprehensive list of documents that illustrated the development and formation of the park.
Information about these specific types of monuments:
CONFEDERATE BATTERY AND BRIGADE TABLETS (ADVANCED POSITION MARKERS)
These tablets are 3’8" x 3’4" in dimensions, with carefully prepared inscriptions cast in raised letters painted in white (contrasting the black background) describing the part taken in the battle by each artillery battery, their position and stating its numbers and losses so far as practicable to obtain. The Union & Confederate tablets are virtually identical. Most of the CS battery markers state the losses were not reported in detail. The tablets are mounted on iron pillars or fluted cast iron posts about 3 feet high, grouted in the ground, and the tablets are inclined at a suitable angle so that the inscriptions can easily be read by persons riding or driving on the avenue. Every tablet is 4’4" in height. The advance position markers were cast by Calvin Gilbert. SOURCE & SOURCE. These tablets usually have accompanying artillery pieces which flank the tablet on either side. There can be as little as one cannon and as many as four. This marker is devoid of guns or cannons. The NRHP reports there should be a cannon on the west side (which there is not) and the other cannon was stolen in the 60s.
The 3rd Massachusetts Battery - US Battery Marker is a contributing feature to the Gettysburg National Military Park Historic District which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Government, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The original National Register Nomination was approved by the Keeper March 19, 1975. An update to this nomination was approved by the Keeper on January 23, 2004. The monument is catalogued as contributing structure number MN552-B.
From the Nomination Form:
Marks position of Walcott's Massachusetts US Battery on July 3, 1863. Narrates events associated w/ Battery during Battle. Located on N side of Wright Ave, W of Taneytown Rd.
Short Physical Description:
MN & 2 Cannons. Painted inscription tablet, 3'8"x3'. Mounted on fluted post, 4'4" high. Cannon on N/S sides of marker, mounted on granite foundation.
Long Physical Description:
N/A
My Sources
1.
NRHP Nomination Form
2.
Stone Sentinels
3.
Virtual Gettysburg
4.
Draw the Sword
5.
Historical Marker Database