Manufacturers AR15 / M16 M4 Carbine Rear Sight Aperture, Used (For Adjustable Sight Models; A2 Style)
Rear Sight Aperture, Used (For Adjustable Sight Models; A2 Style)

This part is found in the following schematics

Rear Sight Aperture, Used (For Adjustable Sight Models; A2 Style)

This part is found in the following schematics