The Hunt’s US Artillery Headquarters Marker on the left or west side of Taneytown Road (State Highway 134) when traveling north along the road. The monument is 342 south of the Hunt Avenue intersection (right turn only). The High Water Mark Walking Trail is on the left or west side of the intersection. There is no monumentation along this stretch of the road. 
Parking is not available along the trail. Also, do not park on Taneytown Road; there is parking available along Hunt Avenue, but that is short term, perhaps a few minutes. I parked about 800 feet away [N 39° 49.003 W 77° 13.970] to the north at the old visitor center parking lot, where parking is free and plentiful. It is here where the trail begins. Most people park here when making the walk. I visited this monument on Tuesday, August 7, 2012 at 12:18 P.M Eastern Daylight Time. I was at an elevation of 557 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
Henry Jackson Hunt (September 14, 1819 – February 11, 1889) was Chief of Artillery in the Army of the Potomac during the American Civil War. Considered by his contemporaries the greatest artillery tactician and strategist of the war, he was a master of the science of gunnery and rewrote the manual on the organization and use of artillery in early modern armies. His courage and tactics affected the outcome of some of the most significant battles in the war. Hunt's most famous service in the war was at the Battle of Gettysburg in July 1863. His new commander, Maj. Gen. George G. Meade, had considerably more respect for Hunt than Hooker did and not only gave him great latitude in directing the artillery, but also used him on occasion as his personal representative. The rest of the war was an anticlimax for Hunt. He served in Virginia to the end of the war, managing the siege operations of Petersburg in 1864 and 1865. He attained the brevet ranks of major general of volunteers and brigadier general in the regular army. SOURCE
The Draw the Sword site helped out by the NPS narrative (and me) offers the following description: One of ten Civil War US Headquarter Markers in the Park. Concrete base, 3’2"sq, 2’7" high. Bronze cannon tube mounted upright on base. Overall 10’6" high. There is cement in the gun which is flush with the muzzle. I was able to see the registry number of the weapon as well as the inspector's initials on then outer muzzle. Bronze inscription tablet & location tablet on West face. Erected by the Gettysburg National Parks Commission. Designed by Emmor B. Cope of the War Department. Colonel Emmor Bradley Cope (July 23, 1834 - May 28, 1927) designed pretty much every tablet for both the Union & Rebel armies, each one distinct, w/ several different varieties as well as the other U.S. Monuments. He is buried in Evergreen Cemetery 
in Gettysburg, as well he should be as this park and most of the things seen here today were designed by him and as such, are his legacy. Completed June 1913. The gun marks the location of Hunt’s US Artillery Headquarters during Battle of Gettysburg, July 1-3, 1863. On the right trunnion are markings which read: Ames CO. Founders Chicopee Mass.
There is an inscription on a front bronze badge which wraps around the front of barrel facing east or the road. The symbol of the Artillery, a pair of crossed cannons is in bronze relief or embossed above the inscription of the badge. The inscription reads.
Headquarters
of
Brig-General
Henry J. Hunt
Chief of Artillery
Army of the Potomac
July 2,3,4,5,
1863
My usual SITE for all things weaponry at Gettysburg usually has the registry information which can be found on the muzzle stamp. I was able to read the Inspector's initials and the registry number but was unable to discern the other information. The information in red is not part of a traditional muzzle stamp but additional information cultivated by my source. That information reads as follows:
Ames ......No. 38 ......1854 ......B.H. ......FDY #6 ......
Alger stands for the Ames Manufacturing Company out of Chicopee, Massachusetts and the foundry responsible for producing this weapon. No. 38 Refers to the Army registration number, a way for the military to keep track of the guns when they take receipt of them from the various Union foundries. B.H. are the initials of the Union Ordnance Officer responsible for inspecting and accepting the final product to make sure it met all standards before the Army took receipt of the weapon. I believe those initials stand for Benjamin Huger (1832-61), a Confederate inspector of ordnance from 1862-64. FDY #6 refers to the foundry's internal control number of the cannon for their inventory purposes only, an accounting system required by the government. 1854 refers to the production date, 1854 of this 1841 model, still being manufactured 13 years after its initial patent.
About the Foundry
Ames Manufacturing Company has its origins in blacksmith and founder, Captain John Ames, who in 1774, made some of the first manufactured shovels in what would emerge as the new, United States of America. The company was founded in 1791 at their factory in Chelmsford, Massachusetts. It was continued by the founder's son by Nathan Peabody Ames (1803-1847) and produced its first swords for the US Government in 1832. A foundry was added, to cast cannon and bells. The historic sword and armament factory, located at 5-7 Springfield Street in Chicopee, Massachusetts, was built in 1847. The structure was added to the National Historic Register in 1983.
Nathan P. Ames was in partnership from 1829 to 1847 with James Tyler Ames. SOURCE
About the Gun (12-pounder bronze Field Guns, Heavy, Model of 1841)
12-pounder bronze field gun, heavy, Model of 1841. Total length, 85 inches; weight specified as 1,800 pounds, but survivors average about 1,760 pounds. Total production, 63; known survivors, 30. Some of these smoothbore guns were rifled with 18-grooves at the beginning of the Civil War. Alger and Ames each made one as 4.62-inch rifles with 12 grooves in 1861. Both survive. NOTE: Tredegar Foundry produced at least eight of these guns as Confederate guns, five of which are known to survive.
SOURCE
About the Inspector
Benjamin Huger (November 22, 1805 – December 7, 1877) was a career United States Army ordnance officer who fought with distinction during the Mexican–American War. He also served as a Confederate general officer during the American Civil War, noted for his controversial performances while in charge of Norfolk, Virginia, and during the 1862 Peninsula Campaign. Relieved of field duty, he would spend most of the remainder of the conflict in staff positions in the Trans-Mississippi Department, where he performed well. After the war Huger took up farming in North Carolina and later in Virginia. SOURCE
The Hunt’s US Artillery Headquarters 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 MN459.
From the Nomination Form:
1 of 10 US Headquarter Markers in Park. Marks the location of Hunt's US Headquarters during Battle of Gettysburg, July 1-3, 1863. Located on west side of Taneytown Road near Hunt Avenue.
Short Physical Description:
Concrete base, 3'2" sq, 2'7" high. Bronze cannon tube mounted upright on base. Overall 10'6" high. Bronze inscription tablet & location tablet on W face.
Long Physical Description:
N/A
My Sources
1.
NRHP Nomination Form
2.
Stone Sentinels
3.
Virtual Gettysburg
4.
Draw the Sword
5.
Historical Marker Database
6.
Wikipedia