198-200 AD., Septimius Severus, Rome mint, Denarius, RIC 138. - wrong illumination

Septimius Severus, Rome mint, 198-200 AD.,
Denarius (18-19 mm / 3,41 g),
Obv.: L SEPT SEV AVG IMP - XI PART MAX , laureate head of Septimius Severus facing right.
Rev.: PROFECT AVGG FEL , emperor on horseback prancing right, holding spear in his right, cloak billowing behind.
RIC IV, 108, 138 (scarce) ; RSC 576 ; BMC 234 ; Coh. 576 .

Curtis Clay 27/06/2010:
"This rev. type passes from the IMP XI PART MAX obv. legend, ... , to SEVERVS AVG - PART MAX. I think that change was simultaneous with Caracalla's change from IMP CAES M AVR ANTON AVG to ANTONINVS - AVGVSTVS, early in 199.
The rev. type, I think, commemorates Septimius' second expedition against Hatra, and the coins, therefore, specify the otherwise unknown exact date of that expedition, spring 199! "