JDW; The 20.5 is the dia of the chamber in MM's just in front of the Rim C'Bore. This converts to .807". Normally on a 12ga chamber this runs to 20.6 (.811) so may be a slightly tight chamber. I believe though, this is rounded to the nearest .1mm so may in fact only be a couple of thousandths small which could be partially accounted for by the shorter chamber. This marking became mandatory about 1893 I believe on all guns given the supplementary smokeless powder proof (PV) & remained in effect until 1924 when the chamber marking was replaced by nominal ga & length in large stylized C, thus C12-65, with the "C" enclosing the 12-65 except for the open right end. 1 436 would be the wt of bbls only in KG (3.166lbs). If you look real close that should be a rearing lion over the PV. PV incidently is Poudre Vive. Someone else will have to give a literal translation, but from a practical standpoint it means smokeless powder proof.
