The Second Corps was also known as Hancock’s Corps. The Corps was commanded by General Winfield Scott Hancock (February 14, 1824 – February 9, 1886), a career U.S. Army officer and the Democratic nominee for President of the United States in 1880. He served with distinction in the Army for four decades, including service in the Mexican-American War and as a Union general in the American Civil War. Known to his Army colleagues as "Hancock the Superb", he was noted in particular for his personal leadership at the Battle of Gettysburg in 1863. One military historian wrote, "No other Union general at Gettysburg dominated men by the sheer force of their presence more completely than Hancock."
The Hancock's U.S. 2nd Corps Headquarters Marker, also known as the 2nd Corps, US Headquarters Marker, is located on Hancock Avenue, on the right or east side of the road if traveling north, along Cemetery Ridge, north of the "Copse of Trees", the site of the High Water Mark of the Confederacy (Pickett's Charge). The tablet is close to the road and faces the opposite direction, to the west, in the direction of the once attacking Confederates during Pickett's Charge. The Brian Farm buildings are just north of this site. This area is an absolute beehive of activity and 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 Monday, August 13, 2012 at 5:52 PM. I was at an elevation of 620 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The monument work was completed under the direction of the Gettysburg National Park Commission (established by the United States Department 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 National Park Service in 1933) SOURCE. According to the NRHP narrative, this monument was built in 1913, which according to my experience is questionable. Other Corps tablets are recorded as being erected in 1912. The HMDB & Draw the Sword sites lists the dedication date as 1906. We'll go with 1913.
Draw the Sword, with descriptive help from the NPS site, offers the following description: A rectangular granite monument, with a large bronze tablets thereon, describing the engagements and movements of each army corps. Trefoil Corps Badge incorporated in center of tablet top. One of the Union army corps monuments, denoting the service of the Second Corps, Army of the Potomac. The inscription on this tablet reads:
Army of the Potomac
Second Corps
Major General Winfield S. Hancock
Brigadier General John Gibbon
First Division Brigadier General John C. Caldwell
Second Division Brigadier General John Gibbon, Brigadier General William Harrow
Third Division Brigadier General Alexander Hays
Artillery Brigade Captain John G. Hazard
July 2. Arrived between 6 and 8 a.m. on Taneytown Road and went into position on Cemetery Ridge on right of 5th Corps and at the left of the Cemetery relieving a part of the 1st. Corps. Caldwell's Division on the left Gibbon in centre and Hays on the right of the line from Cemetery Hill to Round Top. Between 5 and 6 p.m. Caldwell's Division was sent to the support of 3d. Corps and was engaged until sunset. It then returned to its first position.
July 3. At 1 p.m. the Confederate artillery opened a heavy fire all along the line of Hill's Corps and the left of Longstreet's Corps for two hours when an assault under the command of General Longstreet was made by a force of about 15,000 which was repulsed with great loss in killed wounded and prisoners.
Casualties: Killed 66 officers, 731 men, wounded 270 officers, 2924 men; captured or missing 13 officers, 365 men; Total 4369
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 manufactured by 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.
Information about these specific types of monuments:
CORPS HEADQUARTERS MARKERS (FOR UNION & CONFEDERATE TABLETS)
Corps headquarters are similar to division headquarters, consisting of a large, rectangular stone. The difference are the brass tablets set into the stone, which instead of being rectangular like the division tablets, instead have a curved-and-scrolled top, much more ornate & distinctive. I also found the Union Corps tablets to be higher & thicker than their Rebel counterparts.
The union tablets are kind of neat, each individual one having a corps or service branch symbol like the diamond of the Third Corps, the Maltese Cross of the Fifth Corps or my personal favorite, the Trefoil for the Second Corps (This tablet). There can also be found a full moon, a crescent moon, a cross, and a star.
The 3 Rebel monuments are not so bedazzled w/ symbols, rather, at the top they just have written on bronze tablet, Army of Northern Virginia. There are only 3 of the Rebel monoliths compared to 9 of the Union Corps headquarters markers. One of the 9 is for the entire Artillery Reserve so technically there are 8 Corps represented at Gettysburg or 9 depending on your counting.
At Gettysburg, to understand the order of battle & how men were arranged, simply understand the logic & system behind the monuments. Sometimes however, there are weird exceptions which break the pattern & are confusing, making no sense. When I visited the Peach Orchard (a battlefield found here) I found 3 monoliths fashioned in the distinctive Corps design but upon closer inspection I found them to be not for the Union Corps but for the 1st & 2nd Divisions of the 3rd Corps (as well as the one for the entire Artillery Reserve which I can excuse).
There are 4 of these tablets at the beginning of the orchard, all lined up in a neat row. The only reason I can think for them being constructed in the same way & manner as the Corps tablet was to provide uniformity. The 3rd Corps Division tablets are the Birney's Division - US Division Tablet, 1st Division, the Humphreys' US Division Tablet, 2nd Division, the Randolph’s Artillery Reserve - US Division Tablet and the only authentic one, the Army of Potomac. Click the hot links provided to review those waymarks so as to have a clearer understanding. SOURCE & SOURCE
The Hancock's U.S. 2nd Corps Headquarters Marker is a contributing feature to the Gettysburg Nat'l Military Park H.D. which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Gov't, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The monument is designated as structure no. MN453.
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 Hancock's 2nd Corps US Headquarters during Battle of Gettysburg, July 1-3, 1863. Located north side Pleasonton Ave, between Humphery's Avenue and Hancock Avenue.
Short Physical Description:
Coursed, rough-hewn base, 3'2"sq, 2'7"H. Wrought Iron cannon tube mounted upright on base w/ excised Trefoil Corps symbol at cannon base. Overall 10'6"H. Bronze inscription tablet on west side, facing public roads and park avenues.
Short Physical Description:
N/A
My Sources
1. NRHP Nomination Form
2. Stone Sentinels
3. Virtual Gettysburg
4. Draw the Sword
5. Historical Marker Database