I believe that Lefever like most quality gun-makers of that era would chamber a gun to any length desired. Many early 10 gauge guns of the era were made comparatively light weight for use as field guns. I have little doubt that many of these were chambered for the 2 5/8" shell, which was a standard chambering. On the other hand my only 10 gauge Lefever is a 10 lb early gun, no doubt made for waterfowl use & it has 3" chambers. Never to my knowledge a "Standard" chambering, though shells could be special ordered in this length up until around WWI.
Another thing to be aware of is chambers were not as standardized in those days as today & many will run a few thousandths of an inch undersize. As chambers have .04"-.005" taper per inch one can readily see if a chamber is just .001" smaller than the gauge then it will read 1/4" short.
The Lefever you looked at could very well have been chambered for 2 5/8" shells or it could just as well have had a 2 7/8" chamber which was just a bit undersize.
