The unit was commanded by Capt. Edward S. McCarthy who was later killed on June 4, 1864 near Cold Harbor. "He was possessed of a most decided personality; he was rather stout in figure, with a large, full face, piercing eyes, and in manner rather inclined to be reticent in speech; but he had a heart as large as a barn door, was sympathetic with all who needed a friend and as brave as Marshall Ney. Careful of his men under fire, never seeking his own protection, even under the most trying ordeal of a very severe fire from the enemy's guns, such was the character of Captain Edward S. McCarthy." His unit at Gettysburg suffered 2 KIAs, 8 wounded with 25 of their horses put out of commission. SOURCE
The tablet, also called the McCarthy's CS Battery Tablet, is located on South Confederate Avenue, on the left or west side if traveling south, opposite Biesecker Woods. The monument is on the northern part of the road or the 'handle' of S. Confederate Avenue. Further down or south it turns into a ladle like turn and then does it again, resembling a sine wave. The monument faces the west, right into the setting sun. 
Parking is tricky around here so exercise caution. Please do not park on the grass, park on the side of the road. I cannot emphasize that enough! You will be ticketed. I visited this monument on Saturday, March 10, 2012 at 4:18 PM, just before the clocks were set ahead for the Spring. I was at a 600 foot ASL elevation.
These monuments were first built in 1900 and concluded in 1906 according to the NRHP nomination form. One site has the monument being erected in 1912; the NRHP also has it at 1911. 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, with descriptive help from the NPS site, offers the following description: Cast iron tablet, 3’8" x 3’4", with raised inscription painted in a contrasting color and mounted on fluted cast iron post. All 4’4" H. Tablets casts by Calvin Gilbert. The inscription on the 3.8' x 3.4' tablet narrates the events associated with Battery during the Battle. The inscription on this tablet reads:
Army of Northern Virginia
Longstreet's Corps McLaws's Division
Cabell's Division McCarthy's Battery
First Richmond Howitzers
Two Napoleons, Two 3 inch Rifles
July 2 At 3.30 p.m. placed in reserve near here. The rifled guns advanced to this position at 4 p.m. and engaged in severe artillery fight until dark. The men of the Napoleon section sometimes relieved those of the rifle section.
July 3 Advanced and formed part of the main artillery line the rifle section near Emmitsburg Road the Napoleons further to the left all hotly engaged sometimes changing positions. Retired from the front after dark.
July 4 In position near here. One Napoleon aided in checking a hostile advance. All withdrew from the field at night.
Ammunition expended about 850 rounds. One rifle was disabled.
Losses killed 2, wounded 8.
Horses killed or disabled 25.
There are scores of similar monuments for the various Confederate States & Union units which fought at Gettysburg. Four designs represent brigade, division, corps and army headquarters, and each has elements which identify it as Union or Confederate. Many of the tablets were created by Albert Russell & Sons Co. of Newburyport, Massachusetts and are made of granite, bronze and concrete or like this one, cast iron. All of these tablets were designed by architect E.B. Cope. He designed pretty much every tablet for both the Union and Confederate armies, each one distinct, with several different varieties. The monuments 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
Information about these specific types of monuments:
CONFEDERATE BATTERY AND BRIGADE TABLETS (ADVANCED POSITION)
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 brigade, their position and stating its numbers and losses so far as practicable to obtain. They 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
The McCarthys 1st Richmond Howitzers CS 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 identified as structure number MN571-B.
From the Nomination Form:
Civil War tablet that marks position of McCarthy's CS Battery on July 2-4, 1863. Narrates events associated w/ battery during Battle of Gettysburg.
Short Physical Description:
MN & 2 Cannons. Cast iron tablet, 3'8" x 3'4", with raised inscription painted in a contrasting color and mounted on fluted cast iron post. All 4'4" H. Cannon on N/S sides of tablet.
Long Physical Description:
Located on E side of West Confederate Avenue, south of Millerstown Road & opposite Biesecker Woods. The cannon on the north side of the tablet has been removed.
My Sources
1.
NRHP Nomination Form
2.
Stone Sentinels
3.
Virtual Gettysburg
4.
Draw the Sword
5.
Historical Marker Database