I've been struggling with this MM for a few days. It was partially cleaned when I got it. Any help would be appreciated.
I am sure the first 3 letters are TSB. The only option I see is TSBVI. But then the rest of the configuration is off (I think).
The bust types are the same for 122 and 128, the difference between the two is the wreath. RIC 122 has a star at the top of the wreath (see attached pic) while 128 is plain, like yours.
I had one of each but sold the star version (to someone here most likely?) a couple years ago keeping the plain one with remaining silver (needs new photo). 'Plain' ignores the medallion at the top of the wreath but you have to be more of a LRB specialist than I am to care about such matters. RIC points out that the plain issue was made for Constantius II and the star was not so would be earlier, before he was made Caesar. Such things make collecting minor variations acceptable to my way of thinking but, even when there is a reason for the difference, I don't need both in an area not one of my specialties. I'm not sure why I felt the need of either but my collection is not as rational as some.