It is similar to the Mk I Airsporter, Paul, or at least to the sight on a Club model I have. Its smaller (height, which may have been reduced) and slightly wider. If it was a home made sight, why have a counter-clockwise thread when a normal clockwise thread would be so much easier? The sight blade is 'I' section looking from the top and only a perfectionist would have cut away part of it and added the little screw to limit the blade travel. No history came with the rifle it was fitted to as it was an auction buy. I have scanned books and websites but can't see anything that might be it. Saying that detailed pictures of rear sights don't seem to be that common.
thanks everybody for the suggestions received so far, keep them coming.
atb
dogsbody