Trust me I'm about as confused as you are.

It is known that the official 5.5MHz version has a 11MHz XTAL. And the 1st Conchess version is 2MHz with a 4MHz XTAL. Those are correct.

PCB photos of an "S5" version here with a 11MHz XTAL (SteveUK version is also shown):
https://www.schachcomputer-online-mu...-glasgow-1984/

original 2MHz version: https://www.schachcomputer-online-mu...standard-1982/

SteveUK T8 version is 4MHz (R65C02 overclocked to 8MHz is not possible I think)
No photos of real T8 version there.
https://www.schachcomputer-online-mu...msterdam-1985/

Then there's this list:
https://www.schach-computer.info/wik...ss#Modul_Liste

If Sx really was x/2 MHz, then S4(3.2) is 1.6MHz which makes it slower than the default 2MHz module, again makes no sense.