Note that at this point in time chamber length was not marked per se. However the 1 oz maximum load for the shot does indeed indicate a 2 3/4 inch chamber. The nominal 2" chambered game guns carried a 1 1/8 oz maximum mark.
As with many of these topics, nothing is quite so cut and dried.
The ledger shows that the chambers were 2 3/4" and I would take that as fact but the 1 1/4oz. Max. does not necessarily mean that all chambers with that proof were 2 3/4". There are many British guns from that time with 1 1/4oz. proofs which have 2 1/2" chambers and 2 3/4" chambers would have been unusual.
Also, supposedly, guns with 2 3/4" chambers should have the "LC" designation under the 12 in the diamond indicating a "Long Chamber" but I rarely seem to see that mark, even on guns with original 2 3/4" chambers.