NavList:

I wonder if what you have is actually the reflecting prism from an M1 Aiming Post reflector. Here  is a link.

http://www.worthpoint.com/worthopedia/nosunissued-usgi-mortar-aiming-post-m10-reflector

When aiming an indirect fire weapon, howitzers, guns and mortars, you have a sight that you can rotate in azimuth attached to the gun barrel. You set in the desired azimuth of fire and looking through the sight move the barrel in azimuth until you are looking at your aiming stakes that you set out at 50 and 100 meters in line with each other. This gets your tube aimed in the right direction. To be able to see the aiming posts when shooting at night you clamped these reflectors onto the aiming posts and when aiming the gun you shine a flashlight in the direction of the aiming posts and these reflectors let you line up on them. the reason that you must use two aiming stakes is because the sight (called a panoramic telescope, "pantel") is not mounted at the center of rotation of the gun mount so as the gun is rotated the pantel moves which would introduce a parallax error in the azimuth of fire. Using two stakes separated by the same spacing allows you to get around this. The pantel has a horizontal reticle marked in mils. Looking through the scope you establish tha picture with the near post and the far post and the center of the reticle are equally spaced, e.g. the near post on the 10 mark, the far post on the 5 mark and then  the zero mark will be aligned on the original line of the posts which gets your gun accurately aimed.

gl