https://rpc.ashmus.ox.ac.uk/countermark/1436 

Martini, Pangerl 112 a

(AP)RON or (AP)RONI

rectangular 

17 x 6 mm 

Uncertain Sicilian mint (Sicily)

found on Augustus' Tresviral types, this cmk is generally repeated on both sides of the coin. It is found with figurative Sicilian types and can be attributed to the Imperial legate L. Apronius at a time prior to his return to Rome in AD 21. Pangerl lists these as being used in Africa Proconsularis.