Re your query about chamber length stamps at Proof in 19thC - Greener in Modern Shot Guns 1888 refers to designation of "chambers longer than three inches" - these should be stamped with LC and gauge in a diamond. I think that he is referring to 12 bore guns here and probably not to the eight and four bores. This remark was made after his referring to the newly revised Proof Law - 0f 1887