Brokennock--Modern unfired 2 3/4" shells don't measure 2 3/4" either. That's the length of the fired hull . . . although even when fired, they're almost always just a bit short of 2 3/4".
Re dating the gun based on chamber length: Back before 2 3/4" became the standard length for 12-16-20ga guns, American makers didn't mark chamber length. I can't recall ever having seen an
American double marked anything shorter than 2 3/4". A good rule of thumb is to suspect that chambers have been lengthened on an American gun that's not marked 2 3/4" but has chambers that measure 2 3/4". That was standard practice in the gunsmithing community, as American ammo makers dropped the shorter length shells. The only way to know for sure is to access factory records and get a letter on your gun including factory specs from whomever has access to the information.
There were different proof standards for 2 3/4" guns vs those with shorter chambers. I'm guessing those Remington Express 6's are 1 1/8 oz loads. They're almost certainly somewhat hotter than the loads for which the gun was proofed. But we're also talking guns that were built pretty stout, and it's not likely you're going to do much damage to it if you shoot a few of those, on occasion, when you're hunting. If you're shooting a lot, sticking to 1 oz loads is probably a better idea. That being said, my first classic double was a pre-WWII Sauer 16ga, on which I had the chambers lengthened (before I knew better!) And I shot a pile of pheasants with that gun, using those Express 1 1/8 oz loads. The gun survived, but I'd probably be a bit more cautious today.
