I don't think you understand how coins worked back then.

They weren't using currency like we do nowdays, because to the extent that currency was used it was as a convenient way of exchanging precious metals. A mint would put their reputation into guaranteeing that coin had a certain metal purity. Silver would then have been exchanged by the weight of the coins, not by number (some places like attican greece had, for convenience, standardized coin weight).

While a coin with an unknown stamp (and as such an unknown silver purity) would have been slightly harder to spend major markets (and many merchants) would have been qualified to assay coins. Either using a touchstone or through fire assay.