I would say you have a RM125M frame and engine (frame / engine numbers RM125-10001- 30000) and given the CDI is not on the headstock seems to confirm this.
Some say an original M is more valuable than the S because the M was phased out in Australia quite quickly due too the poor performance against the competitors, so there was typically not as many in Australia compared to the S. In saying this, there's not many of either in original condition left anywhere.
Main difference between M and S are:
Hand made pipe (sectional), 34mm Carby (and jetting) mounted via an adapter plate, larger throated airbox, CDI up on headstock, supposedly hand ported barrel (can't confirm), different BTDC timing and maybe different CDI (again can't confirm).