Hello, I've recently come into possession of a 1899 20 Pesetas gold coin. Absolutely everything about the coin checks out (weight, size, markings) except for the rear side which has the wrong mint signature on it (M.P-M instead of S.M-V). M.P-M were minting until 1890. Does any one know if I've fallen for a counterfeit, or an error where the back side of the coin was minted with an old press? And what's it worth, in any case?