The Battle of Gettysburg featured the Union Signal Corps in its role of observing the battlefield. The chief signal officer of the Army of the Potomac, Captain Lemuel B. Norton (1839-1871), had field telegraph trains at his disposal, but did not deploy them. On July 2, the Confederate corps under Lt. Gen. James Longstreet attempted to maneuver into position for an attack on the Union left flank. They were forced into a lengthy counter march, delaying their attack, when they spotted the presence of the Union signal station on Little Round Top mountain and knew that their approach would be reported. During the Confederate assault, the fighting was so heavy that the signal station had to be abandoned until the following day. This tablet commemorates the U.S. Army Signal Corps' contribution to the battle i@ Little Round Top.
The United States Signal Corps Tablet is located on the western slope of Little Round Top overlooking the Valley of Death, Devil's Den & Houck's Ridge. The only way to get to the monument is to start up at the walking trail which skirts Little Round Top and find the trail that moves downward. At one time Sickels Avenue winded around here before it was lowered and straightened. You'll come to the huge New York castle monument and if you reach that you are on your way to finding this monument. Looking down from a satellite view, the entire immediate area is strewn with huge boulders pushed here by the last Ice Age. The location of this are is near the Vincent's Spur on the southern slope of the hill, but this position is more westerly. This monument gives one of the best views, if not the best view, of the 2nd day Gettysburg Battlefield if you peek out around the boulder near the 
Major General G. K. Warren Statue (MN099). When I visited, there was a reenactment going on with troops signaling here to other troops down below in Devil's Den. 
Parking is on Sykes Avenue(RD362) which is 488 feet southeast of this position via the NPS walking trail. One of those trails begins at the Vincent' Brigade Tablet (MN488) located at N 39° 47.446 W 77° 14.197. This tablet is 620 feet from the recommended started point which is far but sensible as you will get to see fourteen other monuments, 6 interpretives and 3 Parrott Rifles. You could go into the crest at the area where the buses park located at N 39° 47.538 W 77° 14.167 located only 123 feet away through a small trail but you would miss all the history Little Round Top has to offer through its monumentation. When parking anywhere on Sykes Avenue, stay off the grass or anything green or you will be ticketed by the Park Police. I visited this site on Monday, July 1, 2013 on the 150th anniversary of the Battle of Gettysburg @ 2:15 PM, EDT & @ an altitude of 615 feet, ASL. As always, I used my trusty and oft abused Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The landform here figured prominently in the battle and the strategies employed to keep or take the mountain. The igneous landforms of Little Round Top was created 200 million years ago when the "outcrop of the Gettysburg sill" intruded through the Triassic "Gettysburg plain". Subsequent periglacial frost wedging during the Pleistocene formed the hill's extensive boulders. "Geologic Resources Inventory Report" (Denver, Colorado: Natural Resource Program Center (NPS). 2009. pp. 12, 16, 24.)
Draw the Sword, using descriptive help from the NPS site and the SIRIS site, offers the following description: Bronze plaque inset into a natural Gettysburg boulder. Marker is a bronze tablet mounted on the northeast side of signal rock. An eagle in relief surmounts the tablet with its talons holding an olive branch and unsheathed arrows. Two signal flags are located beneath the eagle and above an inscription.
It can be exciting to find an original reference in the Gettysburg Park Commission reports which refer to specific monuments. I found the entry for this monument in the July 1,1919 Commission Report sent to the Secretary of War. It reads: Col. E. H. Haskell, of Boston, Mass., and Dr. Fred W. Owen, of Morristown, N.J. (of Gen. Webb’s staff), on May 1, 1919, visited Little Round Top and selected a place on the surface of a large bowlder on the summit to mount a tablet as a memorial to the United States Signal Corps. This tablet has been placed in position and was dedicated with appropriate ceremonies May 16, 1919.....
The monument was dedicated on May 16, 1919 by who I can only assume was the Gettysburg Military Park Commission. The tablet is approximately 4 feet high by 3 feet wide not including the curved portion on top. The inscription reads:
Memorial Tablet
to the
Signal Corps U.S.A.
which
through valiant and heroic
service at
Little Round Top
July 2-4, 1863
and on many historic battle fields
throughout the war of 1861-1865
contributed so greatly to the
success of the Union Armies
this tablet is placed by their
surviving comrades in tribute
to their memory
The United States Signal Corps Tablet 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 MN403.
From the Nomination Form:
Civil War Marker of the War Dept. Era (1895-1927). Tablet commemorates the services of the US Signal Corps during the Battle of Gettysburg. Located on Little Round Top behind General Warren statue.
Short Physical Description:
Bronze tablet, 3'6"x2'1", mounted on NE side of signal rock on Little Round Top. Tablet is surmounted by eagle in relief, its talons holding olive branch & unsheathed arrows. Beneath eagle & above inscription are 2 signal flags.
Long Physical Description:
Marker is a bronze tablet mounted on the northeast side of signal rock. An eagle in relief surmounts the tablet with its talons holding an olive branch and unsheathed arrows. Two signal flags are located beneath the eagle and above an inscription. Located on Little Round Top.
My Sources
1.
NRHP Nomination Form
2.
Stone Sentinels
3.
Virtual Gettysburg
4.
Draw the Sword
5.
Historical Marker Database
6.
Wikipedia