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".
There are Notable exceptions to this "Rule". L C Smith standardized on the 2 3/4" length at a very early date. At some point so did Lefever Arms Co. In their 1913 catalog is stated all 12 ga guns will be shipped with 2 3/4" chambers unless otherwise ordered.
All of my Large Cocking Hook 12 ga Lefevers have 2 3/4" chambers, I seriously doubt they have all been re-chambered.
Neither of these makers in this era marked their chamber length.
