Note the difference in markings between your gun & Skeetx's. His has the 16-65 in the elongated C (chamber) while yours has the 12/C (chamber) in the diamond. It further has the D=65/20.6. This indicates the 65mm (2 9/16") chamber length & the 20.6 is the chamber diameter just ahead of rim seat. This converts to .811" which is proper for a 12ga chamber
As per PeteM's post this type marking was changed to the type on SkeetX's in 1924. It is also noted the D=65/20.6 mark was used only on guns also carrying the nitro (smokeless) proof.
Also note this nitro proof was per European standards & are not the equivelent of those necessary for US SAAMI spec shells. The loads you mentioned are the ones which should be used in this gun.
