https://rpc.ashmus.ox.ac.uk/countermark/1660 

Martini, Pangerl 117

F•A•T

rectangular 

6 x 4 mm 

Uncertain mint in Gaul (Gaul)

as only one example known, uncertain if applied at Nemausus or not. The meaning of the countermarl is uncertain, but Martini suggests if might stand for F(iscvs) A(vgvsti) T(iberii).