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 located on the right or east side of Hancock Avenue (RD310) if traveling north along the road at The Angle. The sculpture is 142 feet southeast from the United States Regulars Monument (MN235), which is on the opposite side of the road. The 121st monument is just south of the Brigadier General John Gibbon Statue (MN802) which is 48 feet to the north of this monument or to the left if facing the monument. The line of monumentation spread out in this area and along the road represents the line of union regiments who defended Cemetery Ridge at The Angle against Longstreet's assault also referred to as Pickett's Charge on July 3, 1863, the final day of the Great Battle. 
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 approximately 3:58 P.M. I was at an elevation of 583 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The Draw the Sword site helped out by the NPS narrative and the SIRIS site offers the following description: Dedicated in 1886, it was originally located on Reynolds Avenue, approximately 1 1/2 miles northwest of its present location. It was moved in 1888 and replaced by the larger monument featured above. It indicates the position held by the regiment on July 2 & 3, 1863. Monument is a three-part granite shaft topped with a granite ball and set on a 3.6×5 foot square base. The lower part of the shaft has a polished face with incised letters. The middle part of the shaft has a polished face with incised inscriptions on all sides and the upper part contains a finished stone obelisk. Overall height is 11.3 feet.
The monument was dedicated July 3, 1886 by the Commonwealth of Pennsylvania. The monument is composed of granite and has the following dimensions: The sculpture is approximately 8 feet 9 inches x 2 feet 5 inches² and the base is approximately 1 foot 5 inches x 6½ inches². There are inscriptions on all four sides which read:
(Front):Col. Chapman Biddle
Brigade Commander
July 1st 1863
P.V. 121 Reg't
Maj. Alexander Biddle
Commanding Reg't.
1st Brigade 3rd Division
First Corps
(Left):Extreme left
of Union line, 1st day
facing west.
Occupied Cemetery Ridge
July 2nd and 3rd.
Erected by the survivors of this
Regiment in memory of their
fallen comrades.
(Back):Called into service by President Lincoln
September 1st 1862. Participated in all
the engagements of the Army of the
Potomac, commencing with the Battle
of Fredericksburg. Mustered out June
2nd 1865.
(Right):Whole number engaged
7 officers 258 enlisted men
Casualties
Killed
12 enlisted men
Wounded
5 officers 101 enlisted men
Captured and missing
1 officer 60 enlisted men
Total 179.
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 MN246.
From the Nomination Form:
1 of 110 Pennsylvania Mn. Indicates position held by 121st Infty July 2-3, 1863. Located E side Hancock Ave S of US Regulars Mn., S of Copse of Trees. Moved from Reynolds Av. 1886.
Short Physical Description:
Mn base 7'x3'2" rough-cut; Shaft: smooth finish taper to 2-part granite monolith, taper w/ incised inscription & topped w/ gable peak - has bas-relief of soldier w/ gun set inside rough cut exterior. Ends w/ circular finished area & incised lettering.
Long Physical Description:
Monument is a two-part granite shaft with a smooth finished taper that is topped with a gable peak that has a bas-relief of a soldier with a gun set inside a rough cut exterior and set on a 7x3.2 foot base. The taper of the shaft has an incised inscription. Designed by A. Pinardi. Located on the east side of Hancock Avenue. Relocated from Reynolds Avenue in 1889.
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