Countermark

https://rpc.ashmus.ox.ac.uk/countermark/1544 
Martini, Pangerl 25
SD (on coins of Gaul)
rectangular 
5 x 3.5 mm 
S(ententia) D(ecurionum) = ‘By resolution of the Decurions’ - the decurions were the local senate. 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 524 Show 524 242 1
RPC I, 525 Show 525 72 1