This is an interesting and often overlooked type, that seems to get lost in the crowd of early to mid 310s base metal folles. It is also rather scarce, only minted at Antioch and for Maximinus II only. AE23mm 5.35g copper follis, minted at Antioch, in 310. IMP C GAL VAL MAXIMINVS P F AVG; laureate head right SOLE INVICTO; Sol, holding globe and making imperial gesture, riding in a quadriga to left; Є under the prancing horses ANT in exergue RIC VI Antioch 142 Specimen is reverse die match with this spec. Interesting orthography, this SOLE instead of the usual SOLI. The look of Sol Invictus is also apart from the usual depictions of an almost naked god (with some drapery), as was the Western interpretation of the Sun God. Here at Antioch, the god is wearing a long traditional garb, with folds hanging over his left hand, in a posture and detail more fitting for an Oriental deity. The SOLE legend might also hint to a lack of familiarity with Latin at this point from the die cutters who worked on the coinage, as earlier types had been known in the East with the usual SOLI INVICTO form (as for instance is the case with the coinage of Gallienus from the mid 260s from Antioch or the one of Aurelian from Cyzicus and Tripolis). An earlier Sol Invictus type minted for Maximinus as Caesar in 309/10 in Antioch shows the also irregular legend SOLI INVICTAE. The SOLE form seems to have originated around 308/9 -- there is a very rare aureus of a previously unknown series minted for Maximinus II as Caesar at Alexandria with this spelling -- and was continued with two base metal series from Antioch and Cyzicus, one ending in 311 and the other in 312. After that, the Sol Invictus standing type ends in the East in 313 with the regular SOLI INVICTO legend form, very likely at the death of Maximinus II. While the design of this coinage is rather pleasing in its familiarity and simplicity, it's probably classicists that might find these irregularities in Latin legends most interesting, for they might be helpful in understanding perhaps just how standardized was "good Latin" at the time and how accepted were variations from the norm, or perhaps how necessary these minute variations were in official scripts. And perhaps if these variations were due to objective situations (like religious ritual incantations -- Sol Invictus is paramount to the Mithraic mysteries) or it was just about the influence of Greek speakers and writers having to deal with Latin at their jobs.
A nice one of these caught my eye sometime in the last couple of months, but went beyond what I was willing to pay. I do have a rather rough example of the earlier type you mention. It came for cheap in a group lot: I believe @Valentinian has a very nice one!
Although RIC does not list this reverse type for Constantine, several specimens have appeared in recent years. Constantine I ("the Great") Bronze nummus Cyzicus mint, A.D. 311-312 RIC 92, var. Obv: IMP C FL VAL CONSTANTINVS P F AVG Rev: SOLE IN-VICTO - Sol, radiate, wearing long robe and holding head of Serapis MKV in exergue; Γ in left field 22 mm, 5.4 g.
SOLE is Sol in the Ablative Case, FYI. From Classics OSU website: The ablative case is the most complex of the cases in Latin. It may be used by itself or as the object of prepositions and it is commonly used to express (with or without the aid of a preposition) ideas translated into English by the prepositions "from" (that is, an idea of separation and origin), "with" and "by" (that is, an idea of instrumentality or association), and "in" (that is, an idea of place where or time when).
I don't think it does. At least not by itself. It might if it's part of a longer invocation/formula but by itself is elliptic in a way that the regular SOLI INVICTO is not. Not to mention that the SOLI INVICTO form is, beyond its clear semantic and grammatical meaning, a canonical expression with a clear use dating back at least to the 2nd century if not earlier. Of course if this specific form is related to a certain Mithraic ritual that has since been lost (for instance), then it might have been recognized by the public (at least part of it, those familiar with Mithras), otherwise by itself, SOLE INVICTO as a stand-alone is a bit confusing.
The lettering on this coin reminds me a bit of this rare aureus of Gallienus with the legend GALLIENAE AVGVSTAE, in this instance an example of the Vocative case. The point is that Roman coins apparently featured acclamations and other infrequent usage of cases beyond the Nominative on occasion.
Maximinus II seems to have been minting some unlikely rarities during his rule in the East. One of them is this underwhelming series of IOVI CONSEVATORI AVGG naming him INVICTUS, a rather ironic title all things considering. The series is dated to 312 at Heraclea, before his crushing defeat in 313 and his subsequent flight further east to Nicomedia and then Tarsus.
