Nice coin, Nikko !
Have you read Lars' paper "Constantine’s vicennalia and the death of Crispus" that discusses these ?
To summarize:
- There are two anepigraphic issues from Rome, one with mintmark SMRA, and one with mintmark SMRP/S/T/Q
- The SMRA issue includes Crispus and therefore dates to the vicennalia celebrations in 326 AD, it has a star in field for the caesars (same as the other mints), not a wreath
- The SMRP/S/T/Q issue excludes Crispus (now dead) and has the diademed bust for Constantine, so must date later. Lars suggests 328-329 AD for Helena's funeral in Rome
So far the SMRA issue has only been seen for Crispus and Constantine II, but presumably it also exists for Constantius II and Constantine himself. Presumably Constantine is laureate on this issue rather than diademed.