Found this in a box of parts and cannot figure out what it fits. No markings and looks to be cast. I have a book on sights but not listed in there. Any ideas? Thanks

I'd guess an Air Pistol.
The Diana 'Bandit' and 'Chaser' air pistols use one very close to that but not an exact match.
Maybe from an earlier model,, or not at all!
That's why it's just a guess..

2142hg may be right. It looks like it belongs to a Chinese made air gun.

looks very close to a Pachmayr Dominator rear sight.

To well made for a Chinese part.

Steve........

early contender? .800 radius straight is the telltale.