Any difference in top-tube length between the P-stem and the S-stem?
Toptube is the distance between the centers of the seat post and the fork steerer.
the mast that goes in the fork , to put the handle bar upon,
makes no change in the one frame they make.
Is that your concern?
the P steering Mast does appear to have a slightly different bend in it,
than the Mast for the M..
but one likely has to Own Both to insure the method of measuring is consistent.