The obverse reminds me of a 16th century Venetian sesino in which case the reverse should be the lion of St. Mark however this coins obverse doesn't look like a lion to me. It could just as easily be anything else, a cross like that is common on medieval coins.
Try reading the reverse letters, if it is a venetian sesino it should say: "+ SANCTVS. MARCVS. VENET.". Size and weight is also important.
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