It's a bolt stop and the body of an adjustable receiver sight, I think, I'll guess that it's intended for the Haenel. At first I thought it might be some sort of sear carrier. I still need to crack a book or do some research to be more certain of it's entirety. I knew that M-S receiver sights in general weren't common, but not the details of them.
You are right, it is a sight base for a Lyman 22 sight. It is intended to replace the bolt stop on either an 88 commission rifle or Mannlicher-Haenel. According to Nick Stroebel's book, it was last produced in 1919. The Lyman 36 sights replaced it. Here is a link to the patent application for this sight.
lyman sight patent
