Just to clarify or perhaps complicate the discussion, my understanding has been that the 12/C proof mark includes chamber lengths up to and including 2 3/4" and the 12/LC (long chamber)
is reserved for 3" and over. Re the chamber length being marked,
I have a W.J.Jeffrey 10 HG that predates this one by about 3 or 4 years and is engraved in the same area with " 3 1/4 10 bore
Perfect Case " This gun has 34" barrels,weighs 11 1/2 lbs, and appears to be built on an 8 ga. frame.As Mr.Jeffrey made quite a few pigeon and wildfowl guns I assume he had a policy of marking chamber length on some of his guns, especially if they were not standard. Terry

Better to keep your mouth shut and be thought stupid,than open it and confirm.