I just won this coin on eBay from one of our members (thanks Genio). It is a Constantine I obverse paired with a Caesar reverse, most likely Constantine II, since he is the only Caesar with this workshop listed. When I see mules I immediately think "unofficial" but compare styles to official issues. Another factor is which part of the Empire did the coin come from. In the West, the production of unofficial coins reached epidemic proportions in the 4th century, while in the East, it happened much less, so Eastern mules are more frequently official mint mistakes. This one looks completely official and I have included an example of Heraclea 60 for comparison.
Genio's description-
"CONSTANTINVS AVG. Centenionalis or nummus, billon, Heraclea, AD321-324, laureate bust of Constantine I right. Reverse, DOMINOR ● NOSTROR ● CAESS, wreath inclosing VOT/V/* , SMHΔ at exergue.
Not in RIC. Hybrid struck with a Constantine I obverse die (RIC.60) and a Constantine II reverse one (RIC.63)."
For more Eastern mules-
a topic on Cyzicus campgates