Is This A Real Or Fake Roman Coin?

Looks like a casting seam on the reverse around 1-2 O'clock

First off, Tyrian shekels are classified as "Greek", not "Roman", even if they were made (or allegedly made) during Roman rule.

Secondly, this isn't actually a Tyrian shekel - it's a precursor, the coin which Tyre's shekel design were originally copied from. The reverse inscription is "BASILEWS ANTIOXOY" - King Antiochus, which makes it a coin of the Seleukid Empire. To identify which "King Antiochus", we need to identify the date. Fortunately, the Seleukids actually put a date on many of their silver coins: in the case of this coin, that would be gamma-qoppa-ro above the eagle's wing, which converts to Year 193 in the Seleukid calendar, which in turn converts to 119 BC, which conveniently lands in the reign of an Antiochus: Antiochus VIII Gryphus.

By 119 BC, Tyre had already seceded from the Empire, and the "Tyrian design" of the eagle was much less commonly struck by the Seleukids - since that was now seen as "Tyre's thing". But they can still occasionally be found, apparently mostly from the Ascalon mint in what is now Israel.

So, is it genuine? I have to say, it looks more "brassy" than a silver coin ought to look, and the patina is very suspicious-looking - but on the other hand, most of the examples of Ascalon tetradrachms I could find seem to have a very similar appearance. Here's one from slightly later in the reign of Antiochus VIII. There also isn't a single example of the type in the FORVM fakes database, which is not definitive proof but rather a good sign that it's not often copied (unlike actual Tyrian shekels).

Silver doesn't rust. So if there's rust on it it's probably made of a metal that does rust, meaning it's a fake.

SAP