here is a well attested fake Postumus being sold, along with other fakes, on eBay by jorjscoins
He also says it is silver.
"Postumus AE Sestertius. Cologne mint, 261 AD. IMP C POSTVMVS PIVS F AVG, laureate, draped, and cuirassed bust left / LAETITIA AVG, galley right with four rowers, SC in exergue.
24.0 gr - 32 mm
SILVER"
