The Meade's U.S. Headquarters Marker is located off of the right or west side of Taneytown Road/Route 134 if traveling south, at the intersection of Taneytown Road (State Highway 134) and Hunt Avenue. The monument is located on the south side of the walking trail which extends between Cemetery Ridge and Meade's Headquarters, on the property of the Leister House. The paved trail leads to the High Water Mark. The front of the monument faces north. 
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 approximately 4:25 P.M. I was at an elevation of 583 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The Draw the Sword site helped out by the NPS narrative (and a little from me) offers the following description: Rough-hewn, coursed, 3 part base, 3’2" sq, 2’7" H. Cannon tube mounted upright on base. The canon has a strong patina as the bronze has long since oxidized. Overall 10’6?H. Inscription tablet on north side and faces public roads and park avenues. The monument was erected by the Gettysburg National Parks Commission. Designed by Emmor B. Cope of the War Department. The monument was dedicated on June 1, 1913.
There is an inscriptions on a front bronze tablet:
(Front):Headquarters
of
Major General
George G. Meade
Commanding
Army of the Potomac
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:
Alger ......No. 11 ......1855 ......B.H. ......FDY #587 ......
Alger stands for the Cyrus Alger Foundry out of Boston, MA and the foundry responsible for producing this weapon. No. 11 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 #587 refers to the foundry's internal control number of the cannon for their inventory purposes only. 1855 refers to the production date, 1855 of this 1841 model, still being manufactured 13 years after its initial patent.
About the Foundry
This armory was founded by Cyrus Alger (born in West Bridgewater, Massachusetts, 11 November 1781; died in Boston, 4 February 1856), Alger was a United States arms manufacturer and inventor. Early in life he became an iron founder, and established his business in Easton, Massachusetts. In 1809 he moved to South Boston, where he founded the works that since 1817 were known as the South Boston Iron Company. He supplied the government with large numbers of cannon balls during the War of 1812, and his works became famed for the excellent ordnance there manufactured. He was one of the best practical metallurgists of his time, and his numerous patents of improved processes show continued advance in the art practiced by him. The first gun ever rifled in America was made at his works in 1834, and the first perfect bronze cannon was made at his foundry for the U. S. ordnance department, The mortar “Columbiad,” the largest gun of cast iron that had then been made in the United States, was cast under his personal supervision. 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 Meade's U.S. 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 MN450.
From the Nomination Form:
1 of 10 Civil War US Headquarter Markers in Park of the War Dept. Era (1895-1927). Marks the location of George G. Meade's Headquarters, commander of all Corps of the Army of the Potomac, July 1-3, 1863. Located in Leister Farm along former Meade Avenue.
Short Physical Description:
Rough-hewn, coursed, 3 part base, 3'2"sq, 2'7"H. Cannon tube mounted upright on base. Overall 10'6"H. Inscription tablet on north side and faces public roads and park avenues.
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