The PP and the PPK have two entirely different frames.
The PP uses two flat panels on either side of the grip area and has steel for a backstrap.
The PPK uses a wrap around grip where the grip IS the backstrap.
The PPK/S model was created specially for the US market to get around the Gun Control Act of 1968 which limited the importation of guns considered to be "too small." The PPK/S model was a PP frame with a PPK slide/barrel.
You need PPK grips. And PPK grips only. No "S"....