These cannons are much smaller than the other cannons used here in 1863 such as the Parrotts or Napoleons. These weapons are quickly identifiable as the tubes are black, very smooth, and almost always in terrific shape and represented at the time, some pretty serious weapon technology. Their trunnions are sometimes devoid of markings, however some examples have stamps on both trunnions. They always read the same: From the guns perspective, the left trunnion reads Patented Dec. 9, 1862 and the right trunnion reads, Phoenix Iron Co.. Their tubes do not have the breech band seen in the similar looking Parrotts, which distinguishes the two. The 3-inch carriages and limbers are also always equally pristine. This is an 1861 model but manufactured a few years later in 1863. I suppose it is possible this cannon saw action here but without the month it was delivered, I cannot know for sure. This model was also mass-produced by the Confederates as well, but not nearly as well made as their Union counterparts. The stamps on the outer muzzles of these rifles are almost always legible and often times pristine which allows tracing back the manufacturing origins a snap. I think this is due to the deep incising of the registry inscription/information along the muzzle as compared to other cannons which were much more shallow in their engraving.
Through my investigations, I learned the man who made the tablets which accompany the cannons also had a hand in restoring these cannons as well. Gettysburg National Military Park has hundreds of these historic artillery pieces on its field of battle. Their carriages, sights, limbers and on rare occasion (should you find them), the caissons have all been restored. Much of the restorative work came from Major Calvin Gilbert who owned the Gettysburg foundry.
Calvin Gilbert owned a Foundry which bore his name in Gettysburg. This man was responsible for creating all of the cast iron battery tablets. His work did not end there as his foundry created many of the carriages for the historic cannons at Gettysburg. Although almost all of the cannons are from the Civil War, their carriages, sights, limbers, wheels and caissons have been restored at his foundry in the early part of the 20th century. Gilbert was in his 70s when he did this work and completed work at other battlefields as well.
The two 3-inch rifles and the Batteries B & L, 2nd U.S. Artillery - US Regulars tablet also known as the Heaton's US Regular Battery Tablet, are located on Pleasanton Avenue on the left or north side of the road if traveling east, just before the Route 134 (Taneytown Road) intersection. Pleasanton Avenue ends here, 108 feet away. The Hummelbaugh Farm is located near this site as well and the maintenance building is across the street. The monument is flanked on either side by 3-inch ordnance rifles. On the right side is rifle no. 824, and on the left side is rifle no. 616; both rifles were manufactured at the Phoenix Iron Company. The monument faces the road in a southernly direction while both rifles face the north. 
Parking is available road-side at intermittently enlarged shoulder cut-outs. Be sure to keep vehicles off the grass or you will be ticketed by park police. I visited this monument on Thursday, July 5, 2012 after 6:00 P.M. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
My SOURCE for all things weapons at Gettysburg provided me additional information about this weapon which did not appear on the muzzle. I used red to designate that data. The rest is as it appears exactly on the muzzle, starting at the top or 12 o'clock position and moving clockwise.
No. 824 ......PICo ......1864 ......S.C.L. ......816 ......FDY #884 ......GRVS 7RH
No. 824 refers to the Army registration number, a way for the military to keep track of the guns. PICo refers to the Phoenix Iron Company in Phoenixville, PA, the foundry where the fun was manufactured. I believe when the Rebels were able to capture one of these guns, they copied it and produced their own, although this one was federally made. 1864 refers to the date of manufacture even though this is an 1861 model. 816 lbs refers to the weight of the firing tube. This number is on the bottom of the muzzle. Each gun is usually very unique and has its own weight which distinguishes it from every other gun, like a fingerprint. Many of these rifles are of the same weight (816 lbs) so the foundry must of had some advanced technology to manufacture rifles with such uniformity. S.C.L. are the initials of the inspector who gave the gun a once over before it was shipped out to the Federal army. The initials stand for the very famous Union Army Inspector Major Theodore Thadeaus Sobibski Laidley. FDY #884 is information not found on the muzzle but provided by the website. This is an internal control number specific to the foundry. The carriages were approximately 900 pounds which makes this entire weapon over 1,700 pounds. The GRVS 7RH, more information provided by my source, refers to number of rifling grooves, left or right twist. In this case, 7 right hand twists were made to rifle this cannon. If you were to look closely at the close-ups of the muzzle stamps, the rifling grooves are easy to discern.
About the Foundry
The Phoenix Iron Works (1855: Phoenix Iron Company; 1949: Phoenix Iron & Steel Company; 1955: Phoenix Steel Corporation), located in Phoenixville, Pennsylvania, was a significant manufacturer of iron and related products during the 19th century and early 20th century. Phoenix Iron Company was a major producer of cannons for the Union Army during the American Civil War. The company also produced the Phoenix column, a significant advance in construction material. Phoenix Iron Works is a core component of the Phoenixville Historic District, a National Register of Historic Places site and in 2006 was recognized as a Historic Landmark by ASM International. SOURCE & SOURCE
About the Inspector
Born on August 15, 1838 in New Hampshire, Lyford graduted from West Point on June 24, 1861 and served with distinction as a Lieutenant Colonel throughout the Civil War. He died on May 9, 1885 while on active duty in command of the Rockford Arsenal, Pennsylvania. He is buried in Section 1, Site 9 at Arlington National Cemetery.
SOURCE I also found this biography: Stephen Carr Lyford (New Hampshire) was commander of the Cairo, Illinois, ordnance depot from February to May, 1862, and chief of ordnance in the Department of Tennessee from July 11, 1862, to September 6, 1863. Lyford returned to West Point as assistant professor of mathematics (1865-1867) and as instructor of ordnance and gunnery (1872). In 1874, he went to Japan, where he presented the emperor with arms and books. At the 1876 International Exhibition in Philadelphia, Lyford was a judge for sporting arms, weapons, and explosives. He died at the Frankford, Pennsylvania, arsenal, which he commanded, in 1885. SOURCEg.cgi?page=gr&GRid=23767841" target="_blank">SOURCE
About the Gun
The second most common rifled field artillery in both Armies generally, and the most common on the Maryland Campaign, the 3 inch Ordnance gun was made of hammer-welded, formed, machined iron. It was popular because of its accuracy and reliability, at least those examples built in Federal shops. Less precise machining and lower-grade iron gave their Confederate counterparts more trouble. Those built by the firm of Burton and Archer were know to be problematic. The 3-inch rifle normally fired Hotchkiss or Schenkel shells that weighed between 8 and 9 pounds. In an emergency it could use 10-pounder Parrot ammunition. It could also be used to fire cannister but, as a rifle, was not as effective with this as howitzers or Napoleons. The maximum range of this weapon was 1830 yards, with the barrel/tube Length 69 inches, the bore 3 inches (hence the name) and the total combined weight (already mentioned above), approximately 1720 pounds. Johnson, Curt & Anderson, Richard C., Artillery Hell: Employment of Artillery at Antietam, College Station, TX: Texas A&M University Press, 1995; and
Schwartz, Peter, Artillery at Gettysburg online. SOURCE & SOURCE.
The tablet marks the position of the Batteries B & L, 2nd U.S. Artillery - US Regulars on July 2-3, 1863 reads as follows:
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Army of the Potomac
Cavalry Corps
First Brigade Horse Artillery
Batteries B & L Second U.S. Artillery
Six 3 inch Rifles
Lieut. Edward Heaton Commanding
July 2 Arrived near the battlefield at 5.30 a.m. and reported to Major General Alfred Pleasanton who ordered the Battery to be held in reserve until near dark when it was moved back two miles on the Baltimore Pike for the night.
July 3 Moved to the front and was ordered to the position occupied the day before but being subject to the severe Artillery fire the Battery was ordered to retire out of range and there remained until the close of the battle.
This area is crazy-stupid with all types of cannons and guns and monuments as far as the eyes (or binoculars) can see, a veritable outside museum. I would suggest allowing up to an hour and a half to walk around this avenue and Hancock Avenue just up the road to fully inspect all the tablets, monuments and weapons in this area.