Lagopus, according to the Kennett article on Belgian proof, the rampant lion over PV is indeed just an indicator of nitro proof. However, under the rules of 1924--and this is a post-1924 gun, by the proofmarks--the standard pressures were different depending on gauge and chamber length: 12,900 psi for standard 12 or 16ga; 14,700 psi for smaller gauges. But if chambers were longer than 2 3/4", then standard proof was 14,700 psi for the 12ga.