FWIW, the circled "16" proofmark on each barrel's flat indicates that each is "short" chambered for the older/first 2-9/16" (65mm) 16ga shotshells made prior to the ammunition upgrades of 1925.
Please do not fire ANY ammo in it, before it's been checked by a certified double gunsmith (not some parts-changer) as okay to shoot - and then never fire any modern 2-3/4" ammo in it, even so-called "low brass" or "field loads".
The reason, besides the age & different/weaker steels used, compared to later/today, is that the crimp of the longer shells, having nowhere to go when the longer shell is fired in a shorter chamber, will partially obstruct the passage of the shot charge, raising pressure.
Altering the chamber to accept the longer shells won't alter the fact that the gun is 90+ years old, and shouldn't be subjected to modern powder's pressure curve - and that safe, low-pressure modern 2-1/2" ammo is currently available from companies like RST, PolyWad, Kent Cartridge, GameBore & others.
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