Thanks for editing my post to show the photos, I tried adding the text for the images but it still would not show??????

I still haven't figured out what the base might be for. At one point I was looking for any rifle with a M53 designation. Only thing I saw was a Johnson Automatic. I have no idea if it has any bearing on this mount though.

The receiver of the rifle this was for would need to slope from proximal to distal end! Very strange.