It is a fake. This type was only struck in gold.
There are too many letters in the obverse for Arcadius. Valentinian II is the best fit. I would looks at possibly SMK[?] or SMN[?]
That doesn't look like PVC damage; which is why the acetone did not work. PVC residue on a coin feels a little greasy (which is why acetone...
my page with some modern countermarks on Roman coins-- http://www.constantinethegreatcoins.com/Countermarked/
from an earlier post in this long thread-- from another post, one of the reasons I am not much interested in going further back than 10 generations
Separate names with a comma.