General rule is as follows:
PPC = Pre-qualification
MPC = Fully spec. qualified, general market flow
SPC = Fully spec. qualified, automotive flow
The same MPC part number may consist of more device masks (revision). SPC part number must be unique, thus it is always the same mask (revision).
I can confirm MPC5566 and SPC5566 are identical parts (if the rest of partnumber is equal) as only one mask exists.