The numbers you have "P16___65.9,75 S.P.28,5 BL" are probably for powder type(S.P.), powder charge & shot charge(28,5 Lead). Are you sure of the letters? Would you have a better pic of the stamps? The caveman with the staff is either a mark for the town of Suhl or a Sauer mark. I have seen it on many Sauer longarms barrels of that time period.

Kind Regards,

Raimey
rse