Another Roman Coin?

This is the series I am studying and pursuing with aggression. There are actually no known varieties of this type struck at Heraclea but it was made at the neighboring cities of Constantinople and Cyzicus. The coin posted is from Rome and the mint mark reads SMRB. This means it could have been Gratian, Theodosius I, or Valentinian II, as Ben noted. We can eliminate Magnus Maximus since only one type is known from Rome (and we don't know what the officina was that struck it). Unless this coin cleans up a little more I am afraid that a definitive answer as to who appears on the front will not be possible. The busts on REPARATIO REIPVB coins remain static while it was being struck from 378 A.D. to 388 A.D. so stylistic differences that could help us nail down an attribution are not present. If I had to take a stab at your coin, Poiano, I would lean towards Valentinian II. I say this because it appears that the letter immediately to the left of the laureate tie on the front of this coin could be a V. That location immediately to the left of the tie is where the name of the emperor always started on this type. If you are curious and want to know for sure, that is the location you should focus your cleaning efforts.