I felt this coin deserved it's on topic instead of adding to my last post. I just received to today and was excited to get it. Now Arles first started out with "ARL" (or "AR") in the ex with P, S and T before to represent the officna (PARL, SARL and TARL [and PAR]). Constantius II restored the the name to Constantia and the ARL (and AR) changed to "CON" (PCON, TCON and SCON). The sizes of the ARL (FH falling horseman) range from 22-23mm and 20-21mm The CON FH range 18-19mm and 16-17mm. This coin is 18mm, but has PARL in the ex. I may be too tired after a 12hr shift, but I believe it's an unpublished transitional piece. Constantius II Ae 18, 3.4 g D N CONSTAN-TIVS P F AVG, diademed, draped & cuirassed bust right / FEL TEMP-REPARATIO, soldier standing left, spearing a fallen horseman who is wearing Phrygian helmet, reaching backwards, PARL in ex. My only CON from Arles and a few of my ARL Arles FH
That looks like it could be a great find. Dane notes another anomaly but under Gallus that could be a counterpart to yours. This one with SARL in exe. The two coins together could point to an unpublished issue. http://www.nummus-bible-database.com/monnaie-29058.htm
