That is a really strange coin. I never seen that combination before--bust of Constantine II, CONSTAN-TINOPOLIS inscription, 2 standards. I wonder if it might be barbarous. Or could it be a typical Constantius II/Constintine II G.E. with a tooled obverse legend, with POLIS originally N CAES?
I agree that the coin being referenced has an interesting look to it. But based on the scans (I have not seen the actual coin) there is no doubt in my mind that the seller is misrepresenting it, and makes of it a fantasy piece. The claim that it was an early prototype of the city commemorative is belied by the small size of the coin. The earliest of these commemoratives ranged in size from 17 to 19 mm across, and 2.75-2.95gm, not the 16mm, 2.03gm of this coin. Let me suggest as a preliminary remark that at this size and weight the coin is too late for 330. The exergue of the coin shows a strange treatment of the first letter of the mint mark, which here is pretending to be the officina mark for a coin of Lugdunum (Lyons). However, for the GE reverse Lyons produced no left facing busts for Constantine II. Before assuming this was specially created at Lyons it would be good to rule out any alternatives. The lettering on the obverse is questionable in more than one place. Without serious examination we cannot rule out an obverse overstrike, but I am dubious that the letters OPOLIS are in the forms as originally struck. What it is I cannot yet say, but that it cannot be a prototype for the city commemorative series I think is beyond question. Let us first consider the alternatives before jumping to such a singular conclusion.