This rare example of the 10-pounder Parrott rifle can be found on the left or east part of South Washington Street/Route 134/Taneytown Road if traveling south, on the opposite side of the road from the old visitor center parking area. The monument is to the left of the stone wall which defines the southwestern border of the National Cemetery. To the right of the monument is this 2.9-inch (10-pounder) Army Parrott Rifle, registry no. 243. Both the monument and the rifle face due west so one would have to face due east to view them. Presumably the rifle points in the direction of the once advancing enemy. This rifle represents the weaponry used by this Battery at Gettysburg, six ten pounders in all. 
Parking is available across the street at the old visitor center. Do not park on the side of the road. I visited this monument on Tuesday, August 7, 2012 at approximately 3:58 P.M. I was at an elevation of 622 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The cannons are in terrific shape: their limbers, carriages, barrels and all other things weaponry are in terrific condition. I noticed both trunnions on this rifle have stamps. The left trunnion reads 10-PDR and the right trunnion reads R.P.P.. My orientation was from the perspective of the cannon not the viewer, or the proper view. The R.P.P. represents Robert Parker Parrott, the inventor of this specific ordnance and the 10-PDR represents the weight of the munitions/shell. Also, the 2.9-inch (found in the title) represents the bore size. They also came in a 3-inch size as well. These rifles are more rarer than the 3-inch ordnance rifles and the Napoleons which seems to be everywhere. The muzzle stamps are practically indistinguishable from one another. The Parrott is very similar to the 3-inch ordnance save for the band around the breech which makes it distinguish it self from the 3-inch ordnance. I noticed several reproductions of these types of cannons made by the War Department at the turn of the century due to a shortage of them.
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 just to the left of the 12 o'clock position and moving clockwise.
No. 243 ......1863 ......W.P.F. ......2.9 ......A.M. ......894 ......GRVS 3RH ...... FDY #93
No. 243 refers to the Army registration number, a way for the military to keep track of the guns when the received a shipment from the various foundries which supported the Union efforts. 1863 refers to the date of manufacture. W.P.F. refers to the foundry, West Point Foundry out of Cold Spring, NY. The foundry is on the National Register of Historic Places. 2.9 is designation of the rifle and refers to the size of the bore in inches. The 10-Pounder designation in the name refers to weight of the projectile which was more like 9.5 pounds. These weapons go by both names. A.M. 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 Alfred Mordecai. 894 refers to the weight of the firing tube or barrel, usually a kind of a fingerprint for these things as each one is unique to the rifle. GRVS 3RH, information provided by my source, refers to number of rifling grooves, left or right twist. In this case, 3 right hand twists were made to rifle this cannon. If you were to look at the close-ups of the muzzle stamps, the rifling grooves are easy to discern. Finally, FDY #93 is an internal control number specific to the foundry.
About the Foundry
The West Point Foundry was an early ironworks in Cold Spring, New York that operated from 1817 to 1911. Set up to remedy deficiencies in national armaments production after the War of 1812, it became most famous for its production of Parrott rifles and other munitions during the Civil War, although it also manufactured a variety of iron products for civilian use. The rise of steel making and declining demand for cast iron after the Civil War caused it to gradually sink into bankruptcy and cease operations in the early 20th Century.
In 1835, Captain Robert Parker Parrott, a West Point graduate, was appointed inspector of ordnance from the foundry. The next year, he resigned his commission and on October 31, 1836 was appointed superintendent of the foundry. It prospered under his tenure, and was the site of numerous experiments with artillery and projectiles, culminating in his invention of the Parrott rifle in 1860. During Parott's tenure, in 1843, the foundry also manufactured USS Spencer, a revenue cutter which was the first iron ship built in the U.S. The foundry's operations peaked during the Civil War due to military orders: it had a workforce of 1,400 people and produced 2,000 cannon and three million shells. Parrott also invented an incendiary shell which was used in an 8-inch Parrott rifle (the "Swamp Angel") to bombard Charleston. The importance of the foundry to the war effort can be measured by the fact that President Abraham Lincoln visited and inspected it in June 1862.
SOURCE
About the Inspector
Alfred Mordecai (June 30, 1840 - January 20, 1920) graduated from West Point in 1861 and fought for the Union. His father of the same name was an ordnance officer of great fame and finding information on him was a snap From the West Point/Cullums Register SITE I learned: Mordecai was a cadet at the Military Academy from July 1, 1857, to June 24, 1861, when he was graduated and promoted in the Army to Brevet Second Lieut., Top. Engineers, June 24, 1861. Throughout his career he served constantly in varying ordnance capacities, eventually as an instructor.
Colonel Mordecai became the Superintendent and Commandant of the Springfield Armory from 1892 to 1895. He retired from active service on January 20, 1904 at his own request, after 40 years' service as a Brigadier-General. He resided in Washington D.C. and died on January 20, 1920 in Washington D.C. at the ripe old age of 80, having lived to see World War I. He is buried in Oak Hill Cemetery, Washington, DC.
About the Gun
The gun was invented by Robert Parker Parrott, a West Point graduate. He resigned from the service in 1836 and became the superintendent of the West Point Foundry in Cold Spring, New York. He created the first Parrott Rifle (and corresponding projectile) in 1860 and patented it in 1861. SOURCE
I also remembered the Antietam Battlefield people placed a small identification marker sticking out of the ground next to one of their Parrotts which read:
10-POUNDER PARROT RIFLE
The cast iron rifle with its band of wrought iron reinforcing
the breech was a modern weapon of the day. It was effective at
moderately long range. The 20-pounder, similar except for size,
was the heaviest rifled cannon used at Antietam.
The most common rifled field artillery piece in Civil War service generally, the 10-pdr. Parrott was made of cast iron with wrought iron breech reenforcement. It was an accurate and reliable weapon. Confederate versions of the Parrott Rifles were built by the Noble Brothers Foundry and the Macon Arsenal in Georgia. This Parrott design went through several improvements during the war and was changed in 1863 to a larger 3-inch bore and matching Parrott shell. By 1864 the 3-inch Parrott was standardized and most of these 2.9-inch guns were withdrawn from service.
About the Battery
The 3rd Battery, New York Artillery was also known as Harn’s Battery. During the battle of Gettysburg, it served as a member of Tompkin’s Brigade in the Sixth Corps, Army of the Potomac. The battery was commanded by Captain William A. Harn (1834-1889). Harn was born in Philadelphia. Captain Harn was dismissed from the service on April 5, 1864, but he was reinstated on April 20. He was mustered out with the Battery on June 24, 1864 and became a light house keeper in St. Augustine, Florida after the War. William Harn died on May 31, 1889 at the age of 55 and was buried in Evergreen Cemetery, Saint Augustine. Under his command, 119 men (along with 6 Parrott rifles) were engaged at Gettysburg with no reported loses.
The monument provides a brief narrative of the 3rd N.Y. Independent Battery Artillery Brigade on July 2-3, 1863 and reads as follows:
(Front):3rd N.Y.
Independent Battery
Artillery Brigade
Sixth Corps
July 2 & 3, 1863
(Back):Forced march 36 miles, second position
No losses
Mustered in May 21, 1861, participating
in all the campaigns of the Army of
the Potomac to the end of the war