With certainty, 1985.
You might find a few in the late 1930’s, but then they were commonly 2 5/8”. Before the 30’s , 2 9/16.”.
Have you checked the chambers table in TPS Barrels chapter ?
Is this of importance to you and if so , why?
Present SAAMI standard extended case length 20ga is 2 5/8 to 2 3/4. The standard was extended in the 1950’s and proof and working pressure standards were increased.
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