This is an unusual coin because of the reverse field marks CI HS. I have heard two theories about the meaning of these letters.
First, that this is a value mark, meaning the coin is worth 100 sestertii. CI = 100:1 HS = Sestertius
Hence the symbol H S or I I S, which is used to designate the sestertius. It stands either for L L S (Libra Libra et Semis), or for I I S, the two I's merely forming the numeral two (sc. asses or librae), and the whole being in either case equivalent to dupondius et semis (Priscian, de Ponder. p1347; Festus, p347, Muller).
also see--
Studies in the Byzantine Monetary Economy c.300-1450, by Michael Hendy (1985) pg 462- 463
However the weights of these coins are the same as the previous issue--group ii in RIC VI with only PLC and no field marks. During this period, CI HS was also only used at the Lyons mint and only for a short period- A.D. 308- 309.
The second theory (abbreviation theory) is that CI stands for Constantine Imperator and HS for Herculius Senior. In A.D. 307 Constantine was officially declared Augustus (or Imperator) and he also married Fausta, the daughter of Maximianus (or Herculius Senior); so this coin might be a way of showing the new title of Constantine and honoring his new father-in-law. If true, it is ironic because this was the last issue before the first outbreak of hostilities between Constantine and Maximianus.
Of course, one of these theories is wrong; and there is always a chance that both are wrong; however it seems more likely that the value mark theory is correct, while the abbreviation theory seems forced. Interestingly, there is more reference to the abbreviation theory on the internet.
Constantine I
A.D. 308- 309
27mm 6.9gm
IMP C CONSTANTINVS P F AVG; laureate, draped bust seen from the rear.
PRINCIPI IV-VENTVTIS; Prince standing facing, head left, in military dress, holding standard in each hand; CI in left field, HS in right field.
In ex. PLC
RIC VI Lyons 299
