My Valerian is billon. I definitely like this one. Valerian I Region, City: Egypt, Alexandria Coin: AEF Billon Tetradrachm AK P LI OUALERIANOC EV EVC - Laurate, beardless bust right of Valerianus. None - Bust of Zeus right, regnal year across field L - Z Mint: (AD 259/60) Wt./Size/Axis: 10.81g / 18mm / - Notes: Dec 6, 13 - Ref:SNG Cop.-, Milne 4011-4012.
If we wanted to have a little consistency in our terms we might consider that Valerian and early Gallienus Antoniniani are decent looking billon while later Gallienus issues need to be silvered to make them look gray. Starting the term Potin for Alexandria at that same time makes sense. IMHO eliminating the term Potin also works. What it really means is 'silver coins without silver' if that makes any sense.
Nice grab for the Diocletian @Mat ! I only have one Potin Tet: RI Carinus 282-285 CE Potin Tet Alexandria Egypt 19mm Athena Seated holding Nike Obv-Rev.jpg