Hi, just curious if there is anything about the coin itself (the "Diocletian" from Antioch) that looks fake to you, other than the fact that it is not supposed to exist and that reverse sure does look like a Jovian maiorina (although I note that most of the ones I've seen seem to have a Christogram on the banner, and this coin does not).
Could this not be either a) a trial strike, b) a mistaken mule because the mint worker was illiterate or c) some sort of medallion instead of circulating coinage. If I wanted to argue c), I would point to the fact that the flan is twice the weight of a normal coin; Diocletian/Galerius won a huge victory over the Persians in 303, late in Diocletian's reign, so a commemorative medallion or similar would not be unexpected, and the reverse type is similar to the reverses struck for Maximinus II a couple years later that were unique to Antioch mint.