Roming Enabled

When I behold the wondrous cross, some of my wondering centres on the sign hanging over Jesus's head, which according to the Gospel of John had something like "Iesus Nazarenus Rex Iudaeorum" written on it, but in art is often shortened to acronym "INRI". I wonder when that convenient representation started to be used? It would have meant nothing to the people who were actually there, after all - and with no Urban Dictionary or mobile phones, the cryptic abbreviation must have remained inscrutable.

Still, the Romans were inveterate abbreviators, so I'm guessing it started quite early. So much of Latin is text-speak.

Wondering this just now (when I should have been marking) the obvious fact presented itself to me - having eluded me for half a century - that the Roman numeral C is short for "centum", and M is short for "mille". Okay, I'm sure you all put II and II together long ago - but still it prompts the question, what do I, V, X, L and D stand for? And if for nothing, why those letters and not others?

Answers on a titulus. [ETA Okay, I see Wiki actually has an article all about this, but I'm definitely open to further, ultra-Wiki thoughts.]