Ok, so I picked up a bunch of old rear sights fairly cheaply. What swung it for me was 2 were clearly 1970s-90s BSA ones. They appear to be metal (is that "Kemetal"?) and both have a riser block below them. Are they from the S variants of the Airsporter or Mercury, or something else?

And if anyone has a good condition rear sight for a Mk5 Airsporter, the one with the integral raised rail thing on the receiver, drop me a line.