A 1913 Lefever catalog has a page devoted to proper loads for the various gauges. For the 20ga loads of both 3/4oz & 7/8oz are given. No mention is made of chamber length. Also I have a post 1907 LAC 16ga with 3" chambers. Although I can in no manner prove it it would be more logical to have been ordered that way than to have been later re-chambered as the 3" never became a "Standard" in 16.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra