There was no chamber mark prior to the 1887 Rules

Under the 1887 - 1896 Rules gauge over C in a diamond was used for 12g chambers 2 1/2”, 2 5/8” & 2 3/4”; LC is 3" or longer
https://www.vintageguns.co.uk/magazine/rules-of-proof-4-london-
I couldn't find the mark used for 20g and 16g greater than 2 1/2"

1896 - 1904 Rules LC indicates 12g chamber 3” or longer & 20g or 16g 2 3/4”
https://books.google.com/books?id=3HMCAAAAYAAJ&pg=PA286