Note that if the gun was made prior to 1887 it will not carry the chamber mark. I have a 12ga side lever W Richards hammer dbl carrying Birmingham marks. It has the normal provisional, final, & view marks with the marking of 13 for bore size. This gun definately does not have Belgian proofs, but also upon looking up William Richards it does not seem to have proper markings for that either. Also prior to 1887 there were no in-between gauge marks so this 13 could well have been nearly up to a 14ga size. It measures slightly over 14 now, but is badly pitted. As there was no chamber mark is hard to say if was originally a 14ga which has been rechambered to 12ga or if was built this way. Either way it now carries a 12ga chamber.

I Didn't Say Everything I Said, Yogi Berra