.........& wouldn't it be more likely to be a 'No5 Mk 1*' (or is it built from a 1941 Mk1 receiver? I somehow doubt it). However, the markings are very professional in appearance & short of handling it in person it would be really helpful to see more photo's of the rifle as a whole. There was what purported to be a toolroom example/prototype in the Charnwood collection. Unfortumately I've no idea what became of it & I can't remember what receiver markings it bore, but IIRC they were not 'standard' type receiver markings as shown here, but were more in keeping with what one would expect on a prototype weapon.