https://rpc.ashmus.ox.ac.uk/countermark/1634 

Martini, Pangerl 82 corr.

X with bar above, all in incuse square

rectangular 

10 x 8 mm 

Carnuntum (Noricum)

Legion X Gemina. see D.W. MacDowall, "Two Roman Countermarks of A.D. 68," NC 1960, 5; BN pl. B, 7 (on this coin type) MacDowall's analysis revealed that this countermark was applied during the Civil War in 68 AD while the legion was stationed at Carnuntum