This coin initially looks like it could be official, but there are some problems. It would be unlisted in RIC...it should be bare headed and should use the shorter legend IM CAE MAGNENTIVS AVG. It is also underweight, but the flan does have its own issues. RIC does footnote examples with this legend, calling them engraver's errors. I am putting this example on my "barb" page; but I sure would like to call it an official unlisted issue.
Magnentius
A.D. 350-3
21x23mm 3.1g
D N MAGNENTIVS P F AVG; rosette-diademed, draped, cuirassed bust right.
FELICITAS REIPVBLICE; Emperor standing left holding Victory on globe in right hand and labarum, A in right field.
In ex. TRS