It appears to be a variant on the coin listed in Sear as number 4345, a Romano-Egyptian tetradrachm of emperor Antoninus Pius, reverse Poseidon standing, Year 5 (=141-142 AD).
As for authenticity, it's not an "obvious fake", and it's not listed on the FORVM Fakes database. It should be about 23mm across and weigh somewhere around 12 to 13 grams.
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