This coin is not listed in RIC without the mintmark, though the footnote on pg 359 says that this issue has been studied and recorded with insufficient care and notes several examples in Vienna and wonders if it is an engraving error or separate issue. Voetter, in the 1921 Gerin catalog, had an example that he listed as London #6.
Lech has several examples--
http://www.notinric.lechstepniewski.info/6rom77bv.htmlMaximianus
A.D. 297- 298
Ӕ fraction
20x21mm 2.6gm
IMP C M A MAXIMIANVS P F AVG; radiate and draped (with or without cuirass) bust right, seen from rear.
VOT XX in two lines in wreath.
cf. RIC VI Rome 77b