Sestertii are large brass coins, usually over 30mm and feature the emperor with a laureate. Yours is a small billon coin and the emperor is featured with a radiate crown making it an antoninianus. The obverse is IMP GALLIENVS then a few letter I can't read, most likely P F, and ends with AVG. The reverse most likely is ANNONA AVG but there is no S C in it, S C = Senatus Consulto ‘by decree of the senate’ and was used on base metal coins, not antoninianii.
I cannot find your exact coin on WildWinds, there are examples of similar antoninianii from the Rome and Siscia mints with the GALLIENVS AVG obverse. Maybe if someone has a RIC catalogue at hand they can check if they can find one with your obverse but if it was my coins I'd just write it off as an RIC 557 variant. Small variations like this are common during the 3rd century.
I collect and deal in ancient Roman coin. In case you're looking for affordable ancient coins or need any help with the coins you already have send me a message.
