Help! Looking For This Exact Effigy On A Constantius II

The coin you are showing is not of Constantius II, the figure is that of Constantius Gallus. If you have been searching for Constantius II that might be part of the problem.

But you are running up against more hidden forces. To get two images that look the same requires a die match. To get that you have to narrow your search to coins from the same mint, which in the case of the coin you show is the mint at Siscia. But that is not enough. At this time (351-355AD), the mint at Siscia was producing this type for Gallus from four workshops (officinae), and the issue was quite prolific. I cannot see the first letter of your mintmark clearly enough to distinguish whether it is alpha, gamma, or delta, though I suspect it is alpha. For the period in question each workshop would have used a great many dies, which were individually cut and not hubbed. So the odds of finding an exact match are not very high.

Yes, the reverse is one of the most common types. The mintmark is indeed where you think it is.

Vcoins

MA-Shops

Please have a look, there are a few that are kinda close but I think there's no exact match here.

AE3 = smaller grade bronze. It is a modern, Latin based designation of coin size. And I think you have determined the right size from what I am seeing.

If all you are looking for is an AE3 "falling horseman" reverse for Constantius Gallus, that would be quite easy. But when you specify that you want an exact match to the obverse engraving, then the large numbers are against you for the reasons I gave earlier. Ancient production methods were nothing like the modern methods. Each obverse die was separately engraved.