This coin is currently for sale on Ebay. The problem is that this type was only noted as a silver miliarense (see RIC VII Sirmium 14). Perhaps this was a special presentation piece, but this does not seem likely as it is not only similar to the silver issue, it is identical- except for being bronze and larger. The patina seems authentic, but that is easy enough to fake, plus with a corroded coin, it is easier to hide that it is not genuine.
Constantine I
Circa A.D. 320
25mm 6.8gm
CONSTANTINVS MAX AVG; Bare head right.
CRISPVS ET CONSTANTINVS CC; Confronted busts of Crispus and Constantine II.
In ex. SIRM
RIC VII Sirmium --