This is a 16g with pre-1925 proof with 2 1/2" chambers
Engelhardt in the 1962
Gun Digest "European Proof Marks, Proof in Great Britain" stated that the 12 over C was not used prior to 1925, which is obviously incorrect, and didn't mention 12 over LC.
I think we can say that gauge over C means "standard chamber length" - 2 1/2" to 3" (after 1925 anyway) and 'LC' means 'long chamber"