Manufacturers AR15 / M16 M4 Carbine Rear Sight Base Index Spring, New (Adjustable Sight Models; 3 Req'd; A2 Style)
Rear Sight Base Index Spring, New (Adjustable Sight Models; 3 Req'd; A2 Style)

This part is found in the following schematics

Rear Sight Base Index Spring, New (Adjustable Sight Models; 3 Req'd; A2 Style)

This part is found in the following schematics