This coin must be from Arles, because the obverse legend was only used there. There are not any Chi-Rho reverses listed in RIC with an A behind the bust. The first series after the elevation of Decentius has an A, but no Chi-Rho reverse. This type must fit right at the end of the first series. The flan is small, so it is not as big as the others from this series, but the style looks fine, so it seems official. It is not in Ferrando either.
Decentius
A.D. 350-3
19mm 3.3gm
D N DECENTIVS CAESAR; bare-headed & cuirassed bust right; A behind bust.
SALVS DD NN AVG ET CAES; large Chi-Rho flanked by A-W
In ex. [TAR]
RIC VIII Arles --
