Code 137 most likely means Serbia. I remember seeing a Romanian diplomatic plate in Loznica but forgot to spot it.
This is because of (personal) assumption that diplomatic codes mostly go alphabetically (136 = Italy, 138 = Japan...), and, as Serbia generally being the successor state of Yugoslavia (Iugoslavia), the code was just carried over to the new state and most likely no new code was assigned, unlike what's generally the case with other former Yugoslav countries.