Ancient Coin - Antoninianus

I'm no expert, but I'm not seeing any bronze disease on this one. It's usually a brighter color and will flake or powder off. That appears to me to just be the patina which shouldn't be taken off. I like the portrait.

The coin looks to be even more interesting than just a standard Postumus coin as it would appear to be one that was struck in the name of Postumus by Aureolus.

My first thought was Aureolus too, definitely struck in Milan. The story is a good one - Postumus had the Gallic empire, usurped from Rome, and Aureolus was a reknowned general (of sorts) for the Emperors in Rome. He made some 'blunders' and knew that soon his head would be on a pike, so he siezed Milan (Mediolanum), hoping to save his own skin by holding the Roman legion sent after him at the walls. Knowing he could not break the siege alone, he tried to enlist the help of Postumus by issuing coinage depicting him from the mint as the siege raged. No help ever came.

Reverse is FIDES EQVIT (loyal cavalry, as Aureolus commanded them). You can tell its Aureolus' issue due to the bust (the celators at Milan had only ever cut the very different imperial bust, so when they captured Postumus' likeness, they did it a bit differently) and due to the legend - most coinage depicting Postumus and with EQVIT in the legend comes from Aureolus' revolt (there may be some regular issues).

Also, coins issued by Aureolus are significantly more valuable. Get that green encrustation off, and you'd have a really good example of the bust.