OM Blackhawk sights

I picked up a 3-screw 1971- ish Blackhawk in 357, unconverted, that is missing the rear sight blade. Is there any difference in an OM vs. a NM sight blade?

Well, without digging any of mine out, I can tell you there is a small difference in some of the rear blades. Some blades have a tiny "bump" protruding at the bottom. It allows you to align the blade in the rear assy slot. Look at your rear sight assy, and look for an extra little cut-out. If it has the cut out, your original blade had the bump. If not, your blade was "smooth." Of you get a rear blade that has the bump & your rear assy doesn't have the slot, then file off the small bump & use it.
I hope I got this clear enough for you.
Don't forget, there is a spring inside the rear hole for the blade to tension against.

Thanks. I'll look for it. This is my first "unconverted 3 screw. Looking forward to shooting it.

Wapiti

I may also add that an old model sight blade does not have the white outline or the groove where a white outline would be painted on around the sight groove. Later new model Blackhawk rear sight blades have a groove stamped around the sight notch--old model and early new model blades are "flat" with no outline groove.
Hope this is understandable!!

Ron