Hmmmm. Looks like I have two no dash 66s in my collection with unusual sights I'll need to have lettered. When I take pictures of them I believe people will see why I'm curious. If the letter response adds anything new I'll post another thread.
The only thing consistent with S&W handguns is their inconsistency...
Off topic a bit, but I've got a nickle M15-3 that also has one of those stainless sights; sn 18K36xx, from around 1977:
Grips obviously aren't original, but I believe that the rear sight is. Reportedly, the gun was a trade-in from an Indian reservation constabulary. No markings, and I've no clue as to which reservation it may have come from. -S2