The tablet is the granddaddy of all the U.S. Regular tablets found here as this is the monument for all of the Corps, Divisions, Brigades and various fighting Federal units that participated here at Gettysburg. This one of a kind marker is a companion piece to the 
Army of Northern Virginia Marker, being almost identical to this marker except in this marker the Federal shield at the top of the tablet is still intact. I noticed both markers are located at the starting point for the final Confederate push on July 3, 1863, also referred to as Pickett's Charge. It was on the plain which separated both armies the Confederates rushed to overtake the Federal position, their fishhook and push them back, thus winning the Battle of Gettysburg. It was Lee's and the Rebel's best opportunity to win the war but it instead resulted in a terrible. crushing defeat to the Confederates which ultimately led to the vanquishing of the South. This tablet was put here, therefore to commemorate so sublime an event.
The Army of the Potomac was the largest Union Army in the Eastern Theater of the American Civil War . Created in 1861, and led first by Brigadier General Irvin McDowell, then by Major Gen. George Mcclellan, and later by Major General John Pope, and Major General George Meade, it fought in many battles, including the Battle Of Bull Run, the Second Battle Of Bull Run, Seven Days Battle, Battle Of Fredericksburg, Battle Of Chancellorsville, Battle Of Gettysburg, and the Appomattox Court House Battle, which effectively ended the war. SOURCE
The Army of The Potomac Marker is located on Hancock Avenue, on the right or east side of the road of traveling north, across from the Angle, a Gettysburg Battlefield and site of the High Water Mark of the Rebellion. The marker faces the west, toward the position of the COnfederate lines about a mile or so away. This area is an absolute beehive of activity as this site represents the best of what Gettysburg has to offer, both historically and monumentally. Parking is plentiful and is available road-side at intermittently enlarged shoulder cut-outs, usually marked with white striping. Be sure to keep vehicles off the grass or you will be ticketed by park police. I visited this monument on Tuesday, August 7, 2012 at 4:46 PM. I was at an elevation of 624 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The monument was built in 1908, according to the NPS/NRHP narrative. The work was done under the direction of the Gettysburg National Park Commission (established by the United States Dept. of War), after they took over the administration of the park from the Gettysburg Battlefield Memorial Association (whose funds had expired) on March 3, 1893, and whose stewardship was then transferred to the Nat'l Park Service in 1933, which entity still operates the park today. SOURCE
Draw the Sword, with descriptive help from the NPS site, offers the following description: One of two large granite markers that were set up with bronze tablets giving the organization of the Army of the Potomac and the Army of Northern Virginia. Shields in bronze will be mounted on the faces of the markers above the tablets. Monolithic monument (4’7?x2’2?x8’9?H) with smooth cut granite face & sides, rough cut back & top. Curvilinear cap. Bronze inscribed tablet on front w/ bronze U.S.A shield above. Stone designed by E.B. Cope, Architect. Civil War Marker that commemorates Army of the Potomac at the Battle of Gettysburg 1863, near the center of that army’s battle line. The inscription on this tablet reads:
Major General George G. Meade
Commanding
The Army consisted of Eight Army Corps
First Corps - Major General John F. Reynolds, Major General Abner Doubleday, Major General John Newton
Second Corps - Major General Winfield S. Hancock, Brigadier General John Gibbon
Third Corps - Major General Daniel E. Sickles, Major General David B. Birney
Fifth Corps - Major General George Sykes
Sixth Corps - Major General John Sedgwick
Eleventh Corps - Major General Oliver O. Howard
Twelfth Corps - Major General Henry W. Slocum, Brigadier General Alpheus S. Williams
Cavalry Corps - Major General Alfred Pleasanton
Artillery Reserve - Brigadier General Robert O. Tyler
July 1. The First and Eleventh Corps arrived and were engaged north and west of Gettysburg and fell back to Cemetery Hill in rear of the town. The Twelfth Corps and a large part of the Third Corps arrived at the close of the day.
July 2. The remainder of the Third Corps and the Fifth and Second Corps arrived in the morning the Sixth Corps in the afternoon. The Third Corps having advanced was attacked by Longstreet's Confederate Corps and Anderson's Division of Hill's Corps. The Fifth Corps and the First Division of the Second Corps going to the support of the Third Corps an engagement ensued until nightfall when the Union forces had been driven from their advanced position and the Confederates repulsed.
July 3. The Twelfth Corps having by order vacated a large part of its line on Culp's Hill on the night of the 2d and Johnson's Division of Ewell's Corps having occupied the works. The Twelfth Corps in the morning attacked and regained the lines it had previously vacated. Hill's Corps and Pickett's Division of Longstreet's Corps in the afternoon attacked the line of the Second Corps and were repulsed with great loss. Stuart's Confederate Cavalry in the afternoon attacked the Second Cavalry Division and the Second Brigade Third Cavalry Division and was repulsed.
There are hundreds of non-sculpted monumentation for the various Confederate States & Union units which fought at Gettysburg. There are about 10 different variations of tablets, markers & monoliths by my reckoning. Six designs represent brigade (2), division (2), corps (2), & army headquarters (2), each different in a subtle way so as to distinguish between armies. Battalion/battery/advance position markers & regimental monuments for U.S. Regulars & Confederate have their own distinct design (2), bringing the total to 10 designs at the battlefield. The bronze tablets were created by Albert Russell & Sons Co. of Newburyport, Mass. The granite bases which accompany the various tablets were created by the Van Amringe Company from Boston, Mass. The cast iron tablets were of course manufactured by the aforementioned Calvin Gilbert. The 1910 Gettysburg Commission report lists the awarded contracts to these companies (not Gilbert) for the tablets. All of these tablets were designed by architect Colonel Emmor Bradley Cope (July 23, 1834 - May 28, 1927). He designed pretty much every tablet for both the Union & Rebel armies, each one distinct, w/ several different varieties. 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. These monuments were erected just after the turn of the century during the first & beginning of the second decade of the 20th century. Each one has since been preserved or restored at least twice since the turn of the 21st century. The plaques & tablets were erected by the Gettysburg Park Commission (established by the War Dept.). The Gettysburg Park Commission is also referred to as the Gettysburg National Military Park Commission or the Gettysburg National Park Commission, to clear up any confusions, especially my own.
On October 1, 1898, the Gettysburg National Park Commission in a letter to the Secretary of War set gave recommendations for continuing the task of organizing & progressing the work of the Gettysburg National Military Park. Every November they wrote their annual report which outlined the work of the GNPC for that year. The following is an excerpt from that report relevant to this waymark. The link at the end of the paragraph will take you to the entire report. The method of marking the positions of troops on this field, as approved by the War Department, is to place the principal tablet or monument of each command at the position occupied by the command in the main line of battle, and to mark the several important positions subsequently reached by each command in the course of the battle by subordinate and ancillary tablets, with appropriate brief inscriptions giving interesting details and occurrences and noting the day and hour as nearly as possible. SOURCE
Most of the Commission reports have been digitized and can be found HERE. The initial 1893 report up to & including 1921 can be found here. 1922 to 1926 are missing & have yet to be discovered. The years 1927 to 1933 were discovered in 1996 and are also included. To my knowledge this is the most comprehensive list of documents that illustrated the development and formation of the park.
About these monument types:
CORPS HEADQUARTERS MARKERS (FOR UNION & CONFEDERATE TABLETS)
The two army headquarters markers are unique on the field. Although they share the curved-and-scrolled brass tablet top with corps markers, their stones are much taller and have a curved top. Both Union and Confederate headquarters have a shield attached separately above the tablet, although they do not always stay attached. This monument is missing its badge at the top. SOURCE
The Army of The Potomac 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 statue is referenced in the NRHP document as structure number MN820.
From the Nomination Form:
Civil War Marker commemorates "headquarters" area of Army of Northern Virginia during Battle of Gettysburg, 1863. Located opposite site where Lee witnessed the repulse of grand assult on July 3, 1863 & opp VA monument. Adjacent W Confederate Ave adjacent to Spangler Woods.
Short Physical Description:
Monolithic monument (4'7"x2'2"x8'9"H) w/smooth cut granite face & sides, rough cut back & top. Curvilinear cap. Bronze inscribed tablet on front w/ bronze U.S.A shield above. Stone designed by E.B. Cope, Architect.
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