I have a 1939 Field Gr. that still has 2 9/16" chambers. I believe that in late 1939 and early 1940 they were changed o 2 3/4". The 20 gauge was changed in the early 1930s to 2 3/4" chambers.

I used a 1/4" piece of aluminum and made a riser for loading 2 1/2" hulls for my Mec 600 Jr.
I had saved a lot of RST 16 guage hulls which at the time were Cheddite hulls and they reloaded nicely.