This is the eBay "consolation prize" I recently mentioned, and just received yesterday.
The reverse type is unlisted, although known from a handful of specimens. The Not In RIC site shows four specimens all from this same reverse die, three for Constantine (2 dies/busts), and one for Maximinus II. My coin is a double die match to the Not In RIC (ex. Lanz) Maximinus II coin.
Although not in RIC, the type was (first?) published in 1959 in "Numizmatičke vijesti", the news journal of the Croatian Numismatic Society, where Ivan Meixner (then president of the society) discussed a specimen from the Stoca hoard. I can't find any other information about this hoard, but am guessing it was from Croatia, although the only similar name I can find is "Gornja Stoca" in what is now Bosnia. The type was mentioned again in a 1969 follow-up article in the same journal, where Meixner now noted the type having been omitted by RIC VI (published a couple of years earlier).
Apart from the AVGGG legend, the reverse type has a couple of interesting features:
1) The emperor on horseback is nimbate! This is the only time Constantine is to be found nimbate on a bronze coin, although there are the facing nimbate solidi from Ticinum a few years later. At this date I'd interpret this as a solar reference.
2) The unfortunate about-to-be-speared fallen enemy on the right is holding a rectangular scutum shield, which would appropriately identify him as a member of the roman army, since this was a civil war.
This type nicely pairs with another equally rare victory type issued at the same time, VICTORA AVGGG (RIC VI Rome 353-354). RIC attests this type from four specimens, two in Paris and two in Vienna. I've never seen one for sale, or anywhere else ... apart from this one that I also bought on eBay a couple of years ago !
I havn't seen the Paris coins, but both Vienna coins are a reverse die link to mine, and their RIC 354 is also an obverse die link), so each of these AVGGG types may have only used one or two dies.
It's notable how rare all the "Milvian victory" types are, perhaps due to this being a civil war where crowing over your fallen fellow citizens would not be well looked upon!