Operating from memory now so correct me if I'm wrong but as I recall the 75mm/17.6 style of chamber mark went out in 1924. This was applied only to guns which received smokeless powder proof & was the length of chamber over the chamber diameter just ahead of the rim seat or the rear of the chamber in other words.
A normal 20 ga chamber would have been expected to measure about 17.7 here so this indicates a bit Snug chamber. As long as the shells enter freely though should present no problem.
Note however, even though this gun was chambered 3" & proofed as such it would not have been intended for modern day 3" Magnum Shells which were not on the market at time of proof. 3" shells were available in the old days but were not heavily loaded. Some desired them for the benefit of extra wadding.
