P for Philips (NXP).
M for Motorola (On Semi), though everyone else makes MMBTs (and MMSD, and so on) too I think, so that one's more genericized I guess.
Other examples: MPS, MPSA, MPSH, MMBTH, etc.
There was a 2N3563 (something like a smaller, faster 3904), which was a TO-106 case (old 70s round plastic blob package), then the PN3563 (TO-92 -- as most PN's were: see also e.g., 2N2222 is TO-18 metal can, whereas the PN2222A is the TO-92 most people think of), and the Motorola MPS3563. Possibly others I don't know.
Or the one popular in avalanche pulse generators: 2N2369 (TO-18), PN2369 (TO-92), PH2369 (Philips/NXP), MPS2369, etc.
Tim