These cannons are much smaller than the other Confederate cannons used here in 1863. The tubes are black, very smooth, in terrific shape and represented at the time, some pretty serious weapon technology. This is an 1861 model but was manufactured in 1862. This model was also produced by the Confederates as well. Both muzzles have well defined stamps which list all the manufacturing information (more on that below).
The two guns/cannons and monument are across from Schultz woods and to the right of the National Guard Armory, on West Confederate Avenue, on the left or east side when traveling south; the marker faces the west. 
This area is Seminary Ridge near the Lutheran Theological Seminary. Parking is rather easy in this area as there are extra wide shoulders. Stay off the grass. There are many monuments and cannons in this area so expect to be here for at least 30 minutes or more to see everything. I visited this monument on Saturday, March 10, 2012 at approximately 11:31 PM, just before the clocks were set ahead for the Spring. I was at 565 feet ASL elevation.
My SOURCE for all things weapons at Gettysburg provided me with the following line of information about this weapon which is also on the muzzle which I modified to make it appear as it is on the muzzle:
PICo - 488 - TTSL - FDY #522 - 1862 - 816 - GRVS 7RH . 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. 488 refers to the Army registration number, a way for the military to keep track of the guns. TTSL are the initials of the inspector who gave the gun a once over before it was shipped out to the army. The initials stand for the very famous Union Army Inspector Major Theodore Thadeaus Sobibski Laidley. FDY #522 is information not found on the muzzle but provided by the website. This is an internal control number specific to the foundry. 1862 refers to the date of manufacture even though this is an 1861 model. 816 refers to the weight of the firing tube. This number is on the bottom of the muzzle. Each gun is very unique and has its own weight which distinguishes it from every other gun, like a fingerprint. 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.
About the Inspector
Theodore Thaddeus Sobieski Laidley (1822 - 1886) was an army officer specializing in ordnance. He was also an inventor and author. Laidley was born in Guyandotte, Virginia April 14, 1822. He graduated in 1842 from the U.S. Military Academy and chose a commission in the ordnance corps and served in the Mexican War. At the end of the war, he returned to the arsenal at Watervliet, New York, as Assistant Ordnance officer. Laidley was put on detached service to write a new ordnance manual published in 1862 which served as an important guide during the Civil War. He served on several ordnance boards designing and testing weapons, taking out eight patents. He was later president of the commission to test the strength and value of all kinds of iron, steel, and other metals at the Watertown, Mass. arsenal 1871-1881, retiring in 1882. Laidley wrote government reports and A Course of Instruction in Rifle Firing, Philadelphia, 1879. Theodore Laidley died in Palotka, Florida April 4, 1886. He was inducted into the Ordnance Hall of Fame in 2008. SOURCE, SOURCE, & SOURCE
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 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, which marks the position of Griffin's CS Battery on July 2-4, 1863 and narrates those events reads as follows:
Army of Northern Virginia
Ewell's Corps Artillery Reserve
Dance's Battalion Griffin's Battery
The Salem Artillery
Two 3 inch Rifles, Two Napoleons
July 1 Reached the field too late to take part in the battle.
July 2 Remained in reserve on this Ridge north of the Railroad.
July 3 The Rifles were moved to this position early in the morning and took part in the cannonade preceding Longstreet's assault and continued firing for some time afterward. Withdrew at night to camp in rear.
July 4 The Napoleons occupied a position on this Ridge south of the Railroad Cut but did no firing. After nightfall they joined the Rifles and with them began the march to Hagerstown.
No losses reported. Ammunition expended 154 rounds.
This area is loaded with all types of cannons and guns, a veritable outside museum. I would suggest allowing up to an hour to fully inspect all the tablets, monuments and weapons in this area.