Looks official to me. I put in a half-hearted vacation bid on it, mainly because Magnentius isn't my focus.
The patina/surfaces don't inspire much confidence, but that's more lack of a positive than a negative.
Of course the bust style doesn't look like other Amiens types, but it is close to early Trier which might confirm Bastien's suggestion that the Amiens mint had been setup using staff from Trier. The DN (vs IM CAE) is interesting - showing a break from Trier even at mint opening, and certainly proving this isn't a Trier die. The small "A" behind the bust is as expected for Amiens.
Assuming the engraver came from Trier, the spread chlamys on the reverse is maybe a little unexpected, but although atypical at Trier it does exist.
The weight also supports this being official - a bit heavy if anything. Size is apparently 22mm (I'd asked the seller).
Were you the buyer, Victor? If so, are you planning to get it authenticated (NGC slabbed ,perhaps) ?
