Apparently, the SAAMI meeting where it was "suggested" that our manufacturers begin marking the chamber length on their shotguns was in 1937, and we begin seeing such markings on 1938 or 39 guns. Some examples from Remington autoloaders, August 1935 --
16-gauge Sportsman Skeet Gun 01 Aug 1935.jpg
May 1937 --
11858 02 May 1937.jpg
April 1943 --
484375 02 April 1943.jpg