Nothing was standardized in the shotshell industry early on. An attempt to do so was started in the late 20's early 30's. Up till then, odd length 410 chambers like the 2 7/8, 1 3/8 , 2 3/8 and others existed. The standards were set at 2, 2.5 and 3 for the 410, though there was nothing in place to demand any mfg to strictly follow that. The ammo companys had as much to do with the standardization as the gun mfg'rs.
I've never heard of a 2 3/4" 410 shotshell. But the marking being on a Stevens shotgun wouldn't suprise me if they mearly left the '2 3/4 inch chamber' marking die in place for all the barrels being made regardless of gauge and then placing the proper gauge marking on the correct barrels.
A 410 then becomes a 2 3/4" chamber (marked) mearly out of need of production speed. You see alot of odd marked stuff from Stevens. Even some Stevens marked guns made by Savage after they took control of them.
The shotshell length is the length of the fired case,,,crimp unfolded. A 3" length shell case is supposed to be 3" in length but in reallity is usually a bit shorter than that and varies from maker to maker. Crimped/loaded length can be all over the map as crimp tightness and center depth can vary alot.
The chamber length is more accurate to spec and will accept any shotshell length up to and including the listed chamber length.
The forcing cone of the chamber (tapered area ahead of the chamber that leads to the bore diameter) is not included in the chamber length dimention.