Watch List notes:

ROMAN & BYZANTINE COINS

Gallienus, 253-268. Medallion (Bronze, 30mm, 19.18 g 12), Rome, 261-262. IMP GALLIENVS P F AVG Laureate bust of Gallienus to right, seen from behind. Rev. MONETA AVG The Three Monetae standing left; all are draped but the central Moneta has drapery and a hair style that differs from the other two; the outer two hold scales with short handles in their right hands and a cornucopiae in their left; the central Moneta holds a long-handled scale and a larger cornucopiae; at the feet of each Moneta to left, pile of coins. Cohen 648. Gnecchi 11. MIR 36, 775. Very rare. With a dark brown patina and remains of ancient gilding. Some scratches, perhaps done in order to give a better surface for gilding, otherwise, good very fine.

Ex Numismatica Ars Classica 72, 16 May 2013, 729 and from the collection of Consul E. F. Weber, J. Hirsch XXIV, 10 May 1909, 2291 (there catalogued as silver).
It seems reasonable to assume that when this coin was catalogued in 1909 it was thought to have been struck in billon, but it does seem most likely that it is just normal bronze.