Well Miller, it might or it might not pass. We don't know until we try. I guess you could submit yours as the "guinea pig" gun, if you want.
But the main issue is that proof pressure back then, at least based on anything I can find, does not = proof pressure now. And as recently as 1940, the heaviest load available for the 20ga was 1 oz. As far as lower velocity, 12ga vs 20ga, according to my book there are current 1 1/4 oz 20ga magnum loads that do at least 1300 fps.
Regardless, if it passes proof--which is a good bit more than just firing one shell, as in the shade tree tire test--then it should be OK (at least the metal parts) with anything currently available. What a steady diet of 3" mags would do to the wood might not be so good. But then a steady diet of even standard target loads might not either, given the age of the wood and what might have happened to it in the many decades since it was made.
