By this point (early to mid 350s) the FEL TEMP REPARATIO fallen horseman coins were minted for 3 people. Constantius II, Constantius Gallus, and Julian II. ...and I think that by the time these coins (with the “M” in the reverse left field were minted, Gallus may have been dead). In any case, of those three, 2 were only Caesars at that point and only one was Augustus. On coins of this era, the Caesars were usually portrayed bare-headed. Only the Augustus wore a diadem on coins. So by seeing the bust wearing a diadem (headwear) on the obverse... by process of elimination, we know it is Constantius II. That’s a tough one for someone new to Roman coins... we would have told ya sooner if you asked. Haha... anyway, I hope you are enjoying this new avenue of collecting!
Thanks for the information directing me to read up more on Constantius II. I maybe could have narrowed it more knowing to note the "M" mint mark, but wondered about it. No, I didn't want to ask because I wanted to see if I could come up with the proper information. There is a lot to be studied with so many coins out there and putting the proper Caesar with each. Time to stock up on more books and read.