This coin was struck with a reverse for Galerius. The proper reverse for Maximinus II has an I in field with a Δ in the exergue.
This series has either an H or an I in the left field because of the Tetrarchic system of belonging to the family of either Hercules or Jupiter, so H was for Hercules and I was for Jupiter. Maximinus II was in Jupiter's family, so the I is correct; but, according to RIC, this series also only used specific workshops for each ruler- first and second were for the Augusti and third and fourth were for the Caesars. So both Galerius and Maximinus II were in the family of Jupiter (I in left field) and Constantius and Severus were in the family of Hercules (H in left field).
So, for this issue, either the wrong reverse was used, or perhaps the workshops were flexible for each pair of Augustus and Caesar- the important part being that the correct god (Hercules or Jupiter) was represented in the field. However, given that all these issues are rated as common in RIC and none of these die pairings were noted by the authors, this seems most likely to be a mint error using the wrong reverse die.
Maximinus II
A.D. 305- 306
25x28mm 9.6gm
GAL VAL MAXIMINVS NOB CAES; laureate head right.
SALVIS AVGG ET CAESS FEL KART; Carthage standing facing, head left, holding fruits in both hands; I in left field.
In ex. B
Cf. RIC VI Carthage 40b