Moneta in Latin does not necessarily mean mint. It was first used as an attribute to goddess Juno, meaning 'the one who warns', and Juno Moneta was the protector of funds and therefore her temple was the place where money was struck in early Rome. Moneta therefore, rather than Mint in general, usually refers specifically to the Temple of Juno Moneta.
During the Middle Ages, moneta became the root for monnaie (French), moneta (Italian), moneda (Spanish), moneda (Romanian), moeda (Portuguese) etc. So moneta was preferred instead of nummum/s (which meant coin and gave numismatics).
So my hypothesis is that MONETA on coins generally means coin and not mint.
Also, I subscribe to Sap's assumption that AG is to be read in conjunction with MONETA and means silver coin.