I know most coins of Constantine are quite common. It is the addition of the letters "DN" on the reverse that makes this coin rare. It is an R5. Billon follis, RIC VII Rome 46 (R5), SRCV IV 16126, Cohen VII -, F, 1st officina, Rome mint, 3.512 grams, 20.1 mm, die axis 0o, 316 A.D.; obverse IMP CONSTANTINVS P F AVG, laureate, draped and cuirassed bust right; reverse SOLI INVICTO COMITI D N, Sol standing left, radiate, nude but for chlamys over shoulders, raising right hand commanding the sun to rise, globe in left, star left, crescent right, R P in exergue
Nice portrait, but why does the "D N" in the reverse legend make this rare? I do like the "RP" mintmark meaning Rome Primus or like you put it 1st Officiana. Overall it's not a bad coin at all.
Here is some information from the seller on the reverse legend. "The reverse die is clearly an older one, re-engraved to accommodate the extra two letters. Even so, they do not fit right and the final N is placed in the exergue. Later in the same year the legend was shortened to SOLI INVICT COM D N and finally abandoned in favor of the traditional SOLI INVICTO COMITI."
Sol Invincible, Companion of Our Lord. Nice find - that's a coin I would buy. I love the small and seemingly unimportant varieties. I have a similar rarity to share, an R6, meaning it is not found in RIC. This particular combination of bust, inscriptions, and field marks is not recorded with Sol and spread chalmys. RIC VII Lyons 62 comes the closest, with Sol standing right, looking left, chalmys over left shoulder. If we rewrote VII, we would need to have a Lyons 62a and 62b. Constantine I AE Follis, 18mm, 2.8g; Lugdunum mint: 316. Obv.: IMP CONSTANTINVS AVG; laureate, cuirassed bust right. Rev.: SOLI INVIC-TO COMITI; Sol standing right, looking left, chalmys spread, holding globe / A - S // PLG Reference: RIC VII Lyons 62 var. The seller of your coin obviously researched it. I found mine in a junk bin, as just another low-grade common Constantine. These are the sort of finds that thrill us variety collectors, and pretty much make everyone else's eyes glaze over in boredom. I heartily applaud your choice!