The authors of RIC vi, as well as Cohen, distinguish between reverse types depicting Genius wearing a modius and towered, although they admit it's sometimes hard to tell. This follis of Galerius as Caesar under Diocletian would be RIC 508b/Cohen 65 if with a modius and 530b/Cohen 83 if towered. I am not familiar enough with these issues from the Trier mint to say one way or the other and I am hoping some of the more knowledgeable members can help me out. Galerius as Caesar, AD 293-305. Roman silvered billon follis, 8.62 g, 27.2 mm, 6 h. Trier, AD 302-3. Obv: MAXIMIANVS NOBIL C, laureate and cuirassed bust, right. Rev: GENIO POPV-LI ROMANI, Genius standing facing, head left, wearing modius(?) or towered (?), naked but for chlamys over left shoulder, holding patera and cornucopiae; S/F//IITR. Refs: RIC vi, p. 196, 508b or p. 197, 530b; Sear 14348.
R.C., Detailing the head on Genius must have been very difficult for the celator . Look at the follis pictured below. I'm sure it's an exaggerated modius like your coin.
Disclaimer... not an expert on this but... I think it's a modius. Towered or turreted has some horizontal and vertical structure to it. A modius sometimes looks like an oil can with an oversized top or just a tall cylindrical hat. Here's a coin from Trier of Licinius that is turreted or towered. Licinius RIC VI 845b
