In terms of pressure, SAAMI (American) standards for the 3" 12ga are the same as for the 2 3/4" 12: 11,500 psi. According to
Kennett's Gun Digest article on Belgian proof, under the rules of 1924: "In the case of guns with chambers longer than standard (2 3/4"), pressures are raised to 14,700 psi and 16,100 psi respectively." (The higher figure is for guns smaller than 16ga.) "Guns passing this proof are stamped on barrels and breech with mark no. 19." That mark is the rampant lion over PV, which is present. So it looks like the gun does have a heavy enough proof to handle the pressure. However, as advised above, with older guns and especially those on the lighter side, you're better off going with lighter loads than 1 7/8 oz--for the sake of the wood, if not the metal.
