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 was a War Department reproduction as there was a shortage of these things to display on the battlefield. Consequently, it does not have any of the tell-tale marking discussed. 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 and worked on the reproductions. 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 3-inch ordnance rifle and the 5th Mass. Battery tablet is located on the left or west side of South Hancock Ave. if traveling north & is surrounded by two 3-inch ordnance rifles. If facing the monument, the gun on the left is no. 612, the one on the right is a War Dept. reproduction. Both rifles point to the west, in the direction of the Confederate enemy along the high ground in the distance. The PA State Monument looms large and is due north not too far away. 
Parking is available at small, cutout shoulders along the road. Be sure to stay off the grass or you will be ticketed by park police. I visited this monument on Thursday, July 5, 2012 in the later part of the afternoon & was at an elevation of 575 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
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(s) and actions of the Massachusetts Light Artillery 5th Battery E on July 2-4, 1863 reads as follows:
Army of the Potomac
First Brigade Artillery Reserve
Massachusetts Light Artillery
5th Battery E
Six 3 inch Rifles
Captain Charles A. Phillips Commanding
July 2 Withdrew at 5 p.m. from the field near the Peach Orchard and went into Battery here.
July 3 About 1.30 p.m. by order of Brig. General H.J. Hunt fired on the Confederate Batteries but did little damage. Opened an enfilading fire soon after on Longstreet's advancing line of Infantry and assisted in repulsing the assault. A charge was made within the range of the Battery immediately afterwards by the Florida Brigade and at about the same time a Confederate Battery opened on the left front which at once received the concentrated fire of the Batteries of the Brigade driving the cannoneers from their guns which they abandoned.
July 4 Remained in this position until afternoon.
Casualties. Killed 2 men. Wounded 1 officer and 13 men. Total 16.
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 Hancock Avenue to fully inspect all the tablets, monuments and weapons in this area.