Need Help Identifying This Roman Coin

Obv:- GALLIENVS AVG, Radiate head right.
Rev:- APOLLINI CONS AVG, Centaur walking right, one forefoot raised, drawing the bow. Z in exergue.
RIC V-I 16

@num, I have a little bit of concern about the green spots by the centaur. Are they chalky or powdery? Or are they hard and adherent? Until you know for sure whether this is bronze disease, you might want to separate this coin from your others.

I think there might still be enough silver in Gallienus's coins for bronze disease not to be a problem.

@spence, it's the latter. Thanks, coin has been quarantined

Roman coins were hammered in large volumes, millions sometimes. The dies used for hammering would last maybe for 10,000 coins, so new dies had to be cut continually. That was an entirely manual labor, and variation between dies for the same type of coin was inevitable. Add to that, that there was likely more than one die cutter during the lifetime of a type. Also, many coin types were minted at several mints across the empire, giving rise to local styles.

The standard database is RIC, Roman Imperial Coinage. It has an online version here: https://numismatics.org/ocre/?lang=en

Its 41,710 entries spanning over more than 500 years of coinage may be a bit overwhelming at first, but it has useful search functions to identify coins (go to Browse). In case of your Gallienus coin, entering part of the inscription (CONS AVG) in the Keyword field, and then Centaur in the same field, yields two entries, one of which is your coin: https://numismatics.org/ocre/id/ric....163?lang=en

In other cases, it may be more tricky to find a match . . .

Billon, denarius, and follis are all different things.

Billon is a material, any low grade silver alloy with less than 50 % silver in it. Your coin has probably around 10 % silver, so it is made of billon.

Denarius is a denomination. Your coin is a double denarius, commonly called antoninianus.

Follis is a term sometimes used to describe a later type of Roman coins made of even lower grade billon or bronze (beginning 294 AD). A more common term for those coins is nummus. (However, neither follis nor nummus are terms used in the RIC database, there those coins are classified according to size and called AE1 to AE4.)