Nice English hammer gun. Out of proof?

[Daryl Hallquist]

Drew, I have a circa 1900 Cashmore Nitro with the 12 over C in a diamond. It is also marked 3" Cases.
Also, here is a Betts with similar markings.

[Drew Hause]

I think the British enjoyed confusing we Colonials. Your directory suggests that the LC was used only after 1925.
The Betts date code looks to be 'O' for 1934-35

Your Cashmore would be 1896-1904 with the 1 1/4 OZ. MAXM

A 1887 - 1896 Cashmore for 3 1/4" cases marked 12 over C.