Thanks AM. My leaning towards a Boulton Paul bracket is the tubular cross bar clamps similar to the mid upper in a Halifax, as for mounting a gunsight I’m thinking that the single hole is the attaching point as per the FN types that hold the clamp around the neck of the reflector sight and then via a single bolt onto the bracket.

It may not be from a turret but the lightweight cast alloy construction does seem to indicate that it might suit an aircraft.