I was gifted an ancient Remington Model 12-A a few years back. It’s in miserable condition. The rear sight is missing and it’s badly pitted. The rear sight is missing.
But it still shoots. By God it still shoots.
L
I think just about any 3/8 inch sight could be fitted to the old dovetail for the rear sight.
Lots of them available for around $20.
I like the marbles bullseye sights.
http://gunwatch.blogspot.com/2015/12/review-of-marble-arms-bullseye-sight.html