A 1913 Lefever Arms Co catalog states that all their 12 ga guns are shipped with 2 3/4" chambers "unless" otherwise ordered. I do not know at what point this took effect, but a Ca 1892 gun of mine SNd 20633 has 2 5/8" chambers while one @ #29,471 has 2 3/4" chambers. All my 12 gauge Lefevers with higher numbers than this one have 2 3/4" chambers. I am thinking, but not certain, they changed to the 2 3/4" chambers about the time they changed to the large single cocking hook.
I am not positive if anyone intentionally bored barrels with 2 5/8" chambers & marked them for 2 3/4" shells other than Parker Bros.
PS; I also have a post-1907 H grade LAC 16 gauge with 2 3/4" chambers. It has regular (Short) forcing cones & there is no indication the chambers have been lengthened, likely ordered that way. It has 28" Twist barrels, full choked in both barrels & weighs in @ 6 3/4 lbs, so likely built as a Duck gun.