Yep, it's definitely a Parthian silver coin, a drachm judging by the strange shape. To confirm the denomination, can you tell us the size and weight?Grifterrec
has some excellent summary pages on Parthian coins. On the basis of the portrait, I think it might be a coin of "the unknown king II", circa 81-70 BC. See This page on Parthia.com
for more examples.
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