Stand by for pedantic drivel:
IHS = The first three letters, majuscule, of ΙΗΣΟΥΣ = Jesus in Greek (IESOUS) --minuscule: Ιησους
The form of the 'S' in Greek shows interesting changes. Sometimes it's like a 'C'. So there's the ancient graffito: IC XC = Iesous Christos - first and last letters of each word. But even in the Greek I was taught, a minuscule sigma at the end of a word is like this: ς
I think that shows the similarity to our 'S' and to the 'C'.
Anyway, all over church art there are variations on the IHS sigil.
In Hoc Signo vincit?