This coin has an extra dot- it is either TRANQVILLITAS dot or dot PTR dot dot. The extra dot in the mintmark seems very unlikely, so maybe it was meant to come after the reverse legend, though I have never seen another example like that...and of course, perhaps it is just an engraving error.
It is also nicely silvered and has a pretty elaborate helmet.
Constantine I
A.D. 322- 323
19mm 3.4gm
CONSTA-NTINVS AVG; helmeted and cuirassed
BEATA TRAN-QVILLITAS; globe set on altar inscribed VO /T IS / XX; above, three stars
In ex. dot PTR dot [dot]
Cf. RIC VII Trier 368
