I understood the other numerals to be "mediaeval", i.e. presumably inventions after the classical period. Chambers's dictionary so names them, e.g. N = 90.
That URL quoted is unclear and in at least one point wrong. If Greek phi was the original symbol for 1000 then it resembled (I), being a stroke through a circle: actually CI) where the ) is a reversed C (so the whole symbol is symmetric). This was actually used, and I) likewise for 500, half of the thousand symbol, again where ) was a reversed C.
I) could change to D. I've always wondered whether (I) was connected with M, because certainly in uncials if you joined the tops but not the bottoms of (I) it would look like an uncial m.
On the other hand C and M are straightforwardly the initial letters of 'hundred' and 'thousand', so it would seem odd if M was derived from some other symbol.
Why phi for 1000? Greek 1000 starts with chi, not phi. Greek didn't use letters like this at all, they used alpha = 1, beta = 2, ... up to 10, then 20, 30, ..., 100, up to the end of the alphabet in natural order (including three obsolete letters). This is the first I've heard of this phi business.
I wonder whether it was originally just M for mille, and gradually got stylized into unusual forms phi and (I), which got carved onto monuments using the letter C forward and reversed. We think of Roman letters as carved but of course hand-brushed forms long preceded carving, so it's not wholly implausible that a rounded M got distorted.
Anyone know anything about this? This is just speculation on my part.
