Antoninianus - possibly Gallienus & hippocampus [решено]

Hello,
Does anyone know which emperor’s reign this is from? I believe it’s Gallienus (I think it’s written on the obverse?) but I’d like to be 100% certain of it.

Also, the relatively smooth surface — as usual — worries me, but I guess it’s just how (cleaned) billon antoniniani are? I don’t even know what metal it is though. The condition seems genuine, at any rate...

diameter: 20.5mm
thickness: 1mm
weight: 2.90g
metal: billon?

Hello,
Indeed, it's an antonianus of Gallienus : it's part of its so call bestiary series. When you see an animal on one side and a radiate bust on the other side, you are almost sure it's this emperor (there are only few exceptions like the eagle for posthumous issues).

https://en.numista.com/catalogue/pieces59469.html

You coin is genuine

I see. I didn’t know there was such a series! The other Gallienus antoninianus I have must be part of that, too — it has a centaur on one side.

And thank you for the reassurance! So it only contains less than 10% silver, that explains why it has such not-silvery appearance.