The other is for the pre-WW2 tang sights, smaller thread.I believe there’s two different threads, but one is very common and I’ve never run into the other.
I think those are for the International and High Power sights. I find all of these old sights very interesting.Walter ,
I'm pretty sure I have a disc I bought by mistake for a Redfield It is A bigger thread diameter and I believe it is for a Lyman!
It is a 5/8 disc with .040" aperture if you can use that it is yours!