I beleive this gun was manufactured between 1950-1958 This is based on it having an elevation adjustment on the rear sight and having Birmingham 4 engraved on the right hand side of the compresion tube.

Does anyone know if replacement breach and piston seals are availble in NZ?   The barrel has a lot of play at the breach and it has to be loosing air presure.   Currently it is only running at 37fps - I could just about throw a pellet faster!   It has resonable cocking presure but  feels a bit clunky so in need of a good clean and re-lube with Moly paste.  Not sure what the piston seal is like as I havn't taken it apart yet, but at the very least it will need oiling but at at least 67 years old I suspect it will also need to be replaced