What number does the letter S stand for in Roman numerals, e.g. C=100? It has been suggested in the Forums that it might mean a 1/2.

I found the following reply:- I recently came across your article on Roman Numerals and the difficulty with the number 1999. Apparently you never were taught medieval Roman numerals. Very few folks seem to know about them--but some dictionaries mention them (my Random House does). Except for A, O, U, and W, all the letters have numbers associated with them--and you will be pleased to learn that Z=2,000. So IZ is a very simple expression for 1999.

And, B=300, E=250, F=40, G=400, H=200, J=1, K=250, N=90, P=400, Q=500, R=80, S=7 or 70, T=160, Y=150, Z=2,000.

https://en.wikipedia.org/wiki/Roman_numerals#Middle_Ages_and_Renaissance

There’s some evidence to support the contention that S = ½.

The sestertius or sesterce (the coin in which the Romans reckoned virtually all sums of money) was abbreviated as HS. This is generally reckoned to be a variant of IIS (i.e. duo et semis or two and a half), because the coin was originally worth two and a half asses.

Lewis and Short’s gigantic Latin Dictionary says that S is used as an abbreviation for several words, including semis (i.e. a half), but I can’t recall seeing it in any surviving texts.