That was a quick escalation to Hitler - just goes to prove Godwin's law! :)
I didn't realize the subtractive style really dates to the middle ages, but that certainly seems consistent with the coins - I checked a bunch more and none seem to use it.
As far as I know the subtractive style was in use in Rome but the purely additive one was more widely used until the Middle Ages. There are also examples where multiplicatives are used but the only thing I can find on Wikipedia is an example from the Middle Ages: "XIII. M. V. C. III. XX. XIII" (13573 = 13x1000 + 5x100 + 3x20 + 13) and a medieval one using superscript: XV ^C XIX (15x100 + 10 + 9)
As for the Hitler one: he just happens to be a widely known person with a family name that changed not so long ago. Plus I actually first learned this from someone joking about whether he'd have been as successful if his father had never changed his name from Schicklgruber. Fun fact: "Hitler" was officially supposed to be pronounced with a long "i" (ee) like Hiedler but apparently Hitler didn't like this pronunciation and suppressed it once in power.
I didn't realize the subtractive style really dates to the middle ages, but that certainly seems consistent with the coins - I checked a bunch more and none seem to use it.