MINTMARK VARIETY. Specimen similar to ROMA 76b except for absence of officina letter. See footnote 2 on p. 359:
"These common coins have been studied and recorded with insufficient care. It appears that there were examples (of which Vienna contains a number) without any officina-letter—a separate issue, or result of error in engraving ?".
Judging from quantity, "a separate issue" is far more probable than "error in engraving". See also note in RIC on p. 126:
"Voetter-Gerin ascribes to London, as Maximian Herculius no. 6, a small radiate issue of the 3 gm. group with rev. VOT XX in wreath, no mint-mark. The attribution seems to be without foundation".
This group probably belongs to the same issue as the specimen shown here.