Mike Ruczynski is the detectorist who unearthed this Roman coin. I was asked to provide a full ID and a valuation.
The coin is a billon (very base silver) follis of Severus II, which was struck when he was Caesar (AD 305-06). On the obverse is the laureate and cuirassed bust of Severus and a legend that reads FL VAL SEVERVS NOB C. On the reverse the standing figure of Genius holds a patera and a cornucopia and is accompanied by a legend reading GENIO POPVLI ROMANI. The mint letters (PLG) directly below Genius stand for Lugdunum.
In Volume IV of David Sear’s Roman Coins and Their Values the reverse type is number 14632 but it is combined with a different obverse. In Roman Imperial Coins the type is listed as number 199A.
The lettering on Mike’s find is weak in places but the coin is otherwise in near VF condition. It’s not a rarity but neither is it as common as some other early 4th century coins of the same denomination. As it stands, a collector might pay up to £50 in order to acquire it.