Chamber length wasn't marked as part of proofing when your gun was made. Neither was load size.
The 12/C mark could be either 2 1/2" or 2 3/4".
Since the flats are marked 2 3/4" and the mark looks old, I'd say the 2 3/4" chambers are original. The maker put that mark their to make the distinction.
The proof house added chamber lengths and load sizes to their marks at later dates to clear these things up.
And Nitro Proof didn't come in until 1896 (from what my reference says).
So that's my thinking ... Hope it helps.
OWD
