The 121st Pennsylvania Infantry served as a member of Biddle’s Brigade in Doubleday’s Division of the First Corps, Army of the Potomac, a Fighting 300 Regiment. The 121st was commanded by Major Alexander Williams Biddle (April 29, 1819 – May 2, 1899). On September 1, 1862, he joined the 121st Pennsylvania Volunteer Infantry, serving under his cousin Chapman Biddle, with whom he fought at the Battle of Fredericksburg, the Battle of Chancellorsville, the Battle of Gettysburg (under Abner Doubleday) and the Battle of Bristoe Station. After the war he retired from Thomas Biddle & Company, and in 1874 he was named a director of the Pennsylvania Railroad. He also served as a director for the Philadelphia Savings Fund Company and the Lehigh Navigation Company among others, and as executor of the estate of James Rush (1786–1869), was instrumental in the erection of the Ridgway Library (part of the Library Company of Philadelphia). Under Biddle's command, the 121st had 306 men engaged at Gettysburg and among them 12 were killed, 106 were wounded and 61 went missing.
The 121st Pennsylvania Infantry Monument is on Reynolds Avenue, on the right or east side of the road when traveling north toward the Lincoln Highway and located on the First Day Battlefield, south of McPherson Woods. 
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 Friday, September 10, 2010 at approximately 5:29 P.M. E.S.T. and @ an elevation of 560 feet, ASL.
The Draw the Sword site helped out by the NPS narrative and the SIRIS site offers the following description: A rectangular monument containing bronze and granite reliefs stands on a low, sloped base. There is bed roll, cap and sword on the top and an American flag is draped over the top right side. There is a bronze rifle, pointing upward, on the left side and a set of small accouterments on the front, sloped part of the base. A bronze relief of the Pennsylvania Coat of Arms is affixed to the lower front of the monument. Monument’s most unique aspect is probably the exploding artillery shell. Flanking markers are apex topped, one foot square. It marks the position the regiment held on the extreme flank of the first corps line on July 1, 1863, until outflanked and forced to fall back to Seminary Ridge.
The sculpture was dedicated July 4, 1888 by the Commonwealth of Pennsylvania. Bureau Brothers were the founder used for this sculpture and Heins and Bye were the fabricator. The sculpture is made of granite with bronze elements and the base is made of stone. The sculpture is approximately 10 ft. 9 in. x 3 ft. 10 in. x 2 ft. 6 in. and the base is approximately W. 6 ft. 9 in. x D. 5 ft. 1 in. There are inscription on all four sides of the monument including founder's mark which read:
(Front):121st Penna. Infantry
July 1st 1863
Occupied this position the extreme left of
the Union line. July 2nd & 3rd on Cemetery Ridge.
Present at Gettysburg 11 officers 286 men.
Killed and died of wounds 20 men
Wounded 5 officers 33 men
Captured and missing 1 officer 60 men
1st Brig. 3rd Div. 1s Corps
(Left):From
Fredericksburg
to
Appomattox
(Right):Recruited in Philadelphia
and Venango Counties
Mustered in Sept. 1st 1862
Mustered out June 2nd 1865.
The 121st Pennsylvania Infantry Monument 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 MN047-B.
From the Nomination Form:
Mark position held by 121 Pennsylvania Infantry on July 1,1863 & did effective service until outflanked on left & forced to fall back to Seminary. 1 of 110 mns in Park to PA commands in Gettysburg Campaign. Located E side of Reynolds Ave at S end.
Short Physical Description:
Mns & 2 flank markers. Smooth base, 6'9"x5'1". Smooth shaft, 3'10"x2'6". All 10'9" high. Relief of draped US flag over corner & exploding shell, I Corps insignia, bronze accoutrements & State seal all on W. Flank marker, apex topped, 1'x1'x1'6".
Long Physical Description:
Monument that has two flanking markers. Monument is a smooth granite shaft 3.10x2.6 foot on a smooth base, 6.9x5.1 foot. Sculptured relief of a draped United States flag is over a corner and an exploding shell, I Corps insignia, bronze accoutrements, and state seal are on the west. Flanking markers are apex topped, one foot square. Located on the east side of Reynolds Avenue at the south end.
My Sources
1.
NRHP Nomination Form
2.
SIRIS
3.
Stone Sentinels
4.
Virtual Gettysburg
5.
Draw the Sword
6.
Historical Marker Database
7.
Wikipedia
8.
Find a Grave