I was right! I was wrong!
My initial identification was incorrect! The coin is indeed Stephen V, as I had thought! Here is the correct information from a Hungarian expert of my acquaintance:
Yes, the letters S-R indicate Stephen V.
These Slavonian denars were the currency of the duke ruling over Slavonia autonomously and bore the characteristic representation of the Patriarchal cross, with two royal heads on each side, on the obverse and a running marten between two stars on the reverse, with MONETA REGIS P SCLAVONIA (moneta regis pro sclavonia, king's coin for Slavonia) around. During the whole time of their validity (c.1255-1384) they retained these figures. Kingdom of Slavonia was only one of titles of a king and usually it was term for the whole Croatia. The area at the time was a Hungarian possession, which was later heavily disputed.
It was a currency coined in silver by the viceroy/civil governor (ban) on behalf of the Hungarian king (and sometimes coined also by Hungarian dukes) and was denominated officially denarius banalis or moneta banalis or simply banalis (banovac in the Croatian language). This silver currency was coined for the first time c.1255 by ban Stephen, from the family Guth-Keled/Guthkeled/Gutkeled. About that time money was forged only by the Royal Hungarian Chamber which gave that right to mint for the first time to Stephen, ban of whole Slavonia between 1248-1260. In 1256, ban Stephen established the first mint of money in Pakrac (mentioned by the name "camera de Puchruch" in king Bela IV's registers). In 1260, the mint was displaced from Pakrac to Zagreb. In the mint of Zagreb the currencies of silver of the Hungarian rulers are coined uninterruptedly from year 1260 to 1384 (as "denarius zagrabiensis"). This money was particularly appreciated due the purity of the silver in which he was coined, by the beauty of its engraving as much as by its quality in general. With this currency it was possible to be bought in all the center of the Europe of that time, because was a convertible currency.
The legends are:
MONETA REGIS P SCLAVONIA (common)
MONETA B REGIS P SCLAVONIA (moneta Belae regis pro sclavonia, scarce)
MONETA DVCIS P SCLAVONIA (scarce)
MONETA REGIS P VNGARIA (rare)
Initials on the Arpad Dynasty coins are:
King Bela IV (1235-1270):
o - o
lily - lily
bird - bird
h - R (Ban Henricus Nemetujvari, 1267-1270)
King Stephen V (1270-1272)
S - R (Stephanus Rex)
R - S (Rex Stephanus)
King Ladislaus IV (1272-1290)
R - L (Rex Ladislaus)
L - R (Ladislaus Rex)
S - L (Ban Stephanus Babonich - Rex Ladislaus, 1280-1282?)
R - R - L (Ban Radoszlav - Rex Ladislaus, 1286-1288)
King Andrew III (1290-1301)
R - A (Rex Andreas)
A - R (Andreas Reax)
S - A (Ban Stephanus Babonich - Rex Andreas, 1300-1301)
R - bird (Rex Andreas - Ban Stephanus Babonich, 1300-1301)
A - bird (Rex Andreas - Ban Stephanus Babonich, 1300-1301)