John, I think i’ve figured out the mystery behind the Bristol’s Lewis and why it has the Norman Vane sight. A photo in the book ‘Pi in the Sky’ which shows the top Lewis installation (and is probably what the museum’s example was based on) has it with the Norman Vane sight. Presuming it’s not easily removeable, it would still be there as Lewis’s with the forward muzzle clip were used on the Scarff ring too, although they didn’t need them, with the Norman Vane sight and the large aiming ring.

This means that the Lewis’s would have been interchangeable with either the Scarff ring or the top Lewis installation (which at least two other 22 Sqn Bristol’s had), which of course makes sense because if you go to the armoury and they didn’t have a Lewis available without the Norman Vane/forward clip, you could be stuck without a Lewis!