Here's an unofficial FEL TEMP. It is interesting that the engraver has an obverse legend that looks like D N CONSTANTIVS P F AVG, which was used for Constantius II, but the bare headed bust looks like Constantius Gallus.
The seller wants to believe that it is an official issue though (or at least hopes that the prospective buyer does) and is asking $505.00 for it. The field mark •S• was used at several mints- Alexandria, Constantinople, Cyzicus, Nicomedia, Sirmium and Thessalonica.
Below is his description--
"Constantius Gallus. as Augustus !! 351-354 AD. Æ Follis. (18mm, 2.57g) CONSTAN-TIVS P F AVG, Bareheaded, draped, and cuirassed bust right. / FEL TEMP RE-PARATIO, Soldier left, holding shield and spearing falling horseman, who raises his hand in defense; shield on ground to right. •S•/ (ALEB) ?. (mint of Alexandria). Unlisted in RIC ----. VF/F, brown patina; nice portrait.
EXCEEDINGLY RARE !! Constantius Gallus was never officially raised to the rank of Augustus; so this coins is either barbaric (quality of workmanship, style and lettering imply this is not the case); die engravers error; or was made in anticipation of him being elevated to Augustus."