Yes, very underweight. Such things occur!
Where did you find the reading TR POT XVIII? I find just TR P XVIII in BMC p. 558, RIC 873, and Cohen 843.
Your coin has bust radiate, cuirassed right. This bust type is not in RIC or Cohen, but is mentioned in BMC from a "note in B.M." Actual specimens with cuirassed bust: CNG E209, 2009, 368 and Künker 174, 2010, 826.