This coin is one from a multiple lot. Would like very much to know what the countermark Δ stands for. You help is very much appreciated. AE 28 Roman Province of Bithynia, Nicomedia (Izmit) 244 - 249 AD RPC VIII unassigned number, ID 19996 Ob.: M IOYΛIOC ΦIΛIΠΠOC AYΓ radiate, draped and cuirassed bust of Philip I the Arab to r. Countermark Δ in front of neck Rev.: NIKOM(HΔEΩN) ΔIC NEΩKOΡ-Ω (of the Nicomedians, twice neokorate) Homonoia standing to left, holding patera and cornucopia.
I suspect it is a valuation/denominational countermark designation. If my hunch is correct, the Δ (delta) served to represent the number 4, and hence designated the coin as having been re-valued to 4 assaria. Such designations on bronze coins in that era were done to assist the fight against economic inflation.
My first guess would be Howgego 788, a denominational countermark. However, most of those were on coins only as late as Gordian III, and most on coins slightly smaller than yours. In his catalog, Howgego notes that group 788 may well comprise more than one group... and he includes some even later, e.g., Gallienus. Your coin, I might say, has an especially handsome countermark. Very nice, and very interesting!
I've seen the 'delta' c/m used as a denominational mark on larger and later coins. One that I recall was a 29mm bronze, of Volusian (about a half-dozen emperors after Gordian III, and just before Gallienus). @cmezner - Here's a note you may want to add to your info files for your nice example: In his book, Coinage in the Roman Economy, Dr. Kenneth Harl briefly discusses this subject on page 141.
As stated above, delta within incuse circle (GIC 670). Here is a similar one on my Cilicia, Seleucia ad Calycadnum - Gordian III.
