The First CS Corps was also known as Longstreet’s Corps. The unit was commanded by General James Longstreet (January 8, 1821 – January 2, 1904) who was one of the foremost Confederate generals of the American Civil War and the principal subordinate to General Robert E. Lee, who called him his "Old War Horse." He served under Lee as a corps commander for many of the famous battles fought by the Army of Northern Virginia in the Eastern Theater, but also with Gen. Braxton Bragg in the Army of Tennessee in the Western Theater. Biographer and historian Jeffry D. Wert wrote that "Longstreet ... was the finest corps commander in the Army of Northern Virginia; in fact, he was arguably the best corps commander in the conflict on either side." Upon my multiple visits to this hallowed battlefield, it is his name I see the most other than Lincoln.
The Longstreet's C.S. Marker - Headquarters Marker is Southwest of Gettysburg on West Confederate Avenue, just past the intersection of Section 4 West Confederate Avenue and Millerstown Road, on the right when traveling south on West Confederate Avenue, near Pitzer's Woods, and located near stop six of the driving tour of Gettysburg National Military Park, across the street from Longstreet Observation tower. The monument faces the east. 
Parking can be had at the observation tower. The last couple or so slots are for buses so don't park there. Please do not park on the grass, park on the side of the road if you are not using the lot. I cannot emphasize that enough! You will be ticketed. I visited this monument on Saturday, March 10, 2012 at 3:43 PM, just before the clocks were set ahead for the Spring. I was at a 585 foot ASL elevation.
The monument was erected in 1907, according to the NPS/NRHP narrative. Another source has it at 1920. It is simply amazing with all the work put into these monuments, not one person had the wherewithal or simple presence of mind to write down the dedication dates or manufacturing dates. I know for sure the work was done 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, which entity still operates the park today) SOURCE. E.B. Cope designed the monuments and every other monolith and bronze tablet at the park. The upright cannon is a 12-pounder Napoleon of Confederate manufacture. The piece was made at the Columbus Arsenal, Columbus, Georgia in 1863.
Draw the Sword, with descriptive help from the NPS site, offers the following description: Erected by the Gettysburg National Parks Commission and completed in 1920-1921; consists of bronze cannon mounted on a granite base. Denotes the approximate location of General Longstreet’s headquarters which were located in the rear of Seminary Ridge. 12-pounder bronze Confederate cannon cemented into granite base, 3’2" sq. 2’7" high with muzzle up. Overall height, 10’6". Bronze inscription tablet on west face. The inscription on the bronze plaques reads:
Army of Northern Virginia
1st Corps Headquarters
Lieut. General
James Longstreet
—————
Division
Major Genl. Lafayette McLaws
Major Genl. George E. Pickett
Major Genl. John B. Hood
July 1,2,3,4,5. 1863
These Headquarters were located at a
school house 900 yards westerly
The Longstreet's C.S. Marker - 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 MN766.
From the Nomination Form:
1 of 4 Civil War C.S. Headquarters Markers of the War Dept. Era (1895-1927). Designates location of Longstreet's Headquarters during Battle of Gettysburg, July 2-3, 1863. Located on W side of Section 4, West Confederate Avenue, S of Millerstown Road.
Short Physical Description:
12-pounder bronze Confederate cannon cemented into granite base, 3'2" sq. 2'7" high w/ muzzle up. Overall height, 10'6". Bronze inscription tablet on N face.
Long Physical Description:
N/A
My Sources
1.
NRHP Nomination Form
2.
Virtual Gettysburg
3.
Draw the Sword
4.
Historical Marker Database
5.
Wikipedia