Watch List notes:

The Roman Empire
Nero augustus, 54 – 68

Aureus December 57-58, AV 7.61 g. NERO CAESAR AVG IMP Youthful bare head r. Rev. PONTIF MAX TR P IIII P P around oak wreath enclosing EX S C. C 208. BMC 15. RIC 14. CBN 20. Calicó 423.
Well struck in high relief and about extremely fine

Ex Roma Numismatics sale II, 2011, 589.

The legend EX SC appears on many coinage types of Nero during his early reign. The inference can be to the senate consecrating the acts of, or for, Nero. Given that Britannicus was still in the background it would have been important to Nero that the senate be behind his actions.