In post #19 those look like interchangealbe peep site apetures. The Lyman gun sight company and others like Williams used to sell lots of this kind of precision "open sight" hardware. For iron sight high power matches these are great. You choose the aperture based on distance, target Bull size, and light conditions. I have a set that works on a Williams sight on an 03A3 Springfield. Probably the most accurate rifle that I own.