Well that's interesting. That is the basic S&W "single screw" sight, but it has been modified to permit some other form of sighting attachment to be added to the movable block. The nature of its mounting means that whatever the fixture was, it could be adjusted for height, but not side to side. I wonder if it was a peep sight. I have seen perforated disk sights that were mounted on some prewar revolvers.

I suppose whatever attachment was mounted there could have had its own lateral adjustment mechanism built into it.

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David Wilson