WWII-era M15 Rifle Grenade Launcher Sight “mounting plate”

This is an original U.S. WWII-era M15 Rifle Grenade Launcher Sight mounting plate, (also referred to as the M15 sight or grenade launching sight), complete with its accompanying War Department technical bulletin (TB 23-30-1, dated March 13, 1944) providing installation and usage instructions.

The square metal piece in your photos (the black rectangular plate with two screw holes, partially unwrapped from the preservative paper) is the mounting plate (also called the base plate or bracket) for the WWII U.S. M15 Rifle Grenade Launcher Sight.

Its purpose:

•  It permanently attaches to the left side of the rifle’s wooden stock (on the M1 Garand, M1903 Springfield, or M1 Carbine) using two special wood screws (included in the kit).