Hi PeteDeal,
the shotgun chamber length at that time was 16/65.It's very
easy to measure.You measure with a caliper the diameter at the end of the chamber and than you cut from a stiff paper one piece with the measured width and a length of ca. 76mm=
3 inch and insert the paperstrip into the chamber.When you feel some resistance you mark with a pencil and than maesure again with caliper and you know your chamber length,
regards Johann