As regards the C or LC in the diamond, at least Nigel Brown in his British Gunmakers vol. 1 p. 427 says in referring to the period 1887-96 that "initials LC indicated a long chamber of at least 3 inches."
I believe this to be correct. In fact the way I read the definition in the proof law it seems to state the LC was to be applied to any chamber "Longer" than 3". There are however known guns having 3" chambers both with & without the LC mark. Seems as if some of the proof house workers themselves were not absolutely certain as to whether or not to apply it to a 3" chamber, but I have no knowledge of its having been used on a chamber shorter than 3".
