This is from a 1960's era Beretta Silverhawk SxS 20ga The barrels or rib are not marked for chamber length, only gauge.
Is the "70' on the barrel bottom indicative of 70mm, 2-3/4" or something else?

17.4 mm = .685" (front) chamber dimension; normal 20g
70mm = 2 3/4"

Thanks Drew.

The old Silverhawk was essentially Beretta's 409 but tweaked for the American market (full pistol grip and beavertail). The 70 mark confirms it, but they were all 2 3/4"--unless they were the 10, 12, or 20ga magnum version. Those were 3".