Argo,
I'll remind everyone again, just as the list above advises-There are many exceptions.

I've handled at least a dozen different grades of genuine Darne guns that had serial numbers between 50 and 60 thousand. Hell, I've owned two of them. I have always hazarded a guess that they were pre 1923, based on old style proof marks and the odd (for Darne production) all digit serial numbers, as opposed to the letters/numbers more commonly seen.

My oldest Darne at the moment is the 6.5cm marked Halifax, which, would be considered to be in remarkable condition for a gun of pre 1912 vintage, and, one that does not seem to be cataloged in that era. I'll mention again that it has a 4 digit serial number, and has components that are marked with several different serial numbers from around the same sequence. I have no idea as to why this is. I don't find the catalog issue to be deeply troubling, the Halifax grade guns seemed to fade in and out depending on the economic times. There were three grades of Halifax just prior to WWII, fewer when economic times were booming.

All of the French guns I have seen that were marked for 2 3/4" chambers were modern production. I have handled guns that were stamped 65 that actually had 70mm chambers, and can document they were built with the 70mm chamber. The big switch seemed to occur in the early 1950s.

I see no reason to doubt that some were built and proofed that way prior to that becoming standard practice.

Best,
Ted