Looking at the coin in hand, is there any chance that it reads FRVG•F rather than FRVGF?
I just got home and took another look at it with a strong light and I don't see any • or remnant of such.
Nevertheless I think Carausius must be on the right track. If you look at a few dozen semuncial asses, Crawford 337 to 344 (not counting the well made VER OGVL GAR series) you'll find masses of different legend and style varieties, and as regards legends, pretty much an absence of clear • periods. The F may have been intentional and the legends on these coins must I think read FRVG F, and the error is that it should read FRVG L F (Lucius Fiius).
This is actually consistent with some of the silver denarius varieties where L PISO FRVG L F is seen. It is an error but the error is the omission of L rather than the addition of F. There are dozens of much worse errors than this in the semuncial bronze series, and no evidence they discarded dies due to them!
The Romans weren't great on their use of periods. Still this is a very interesting legend variety.