I don't know about PPS, but first issue left-wheelers (New Armys/New Navys) have the assembly number on the inside of the grips. If the assembly number is four digits the grip number is the last three digits of that number.
It seems likely that on your PPS the number on the grips would be the last three digits of the serial number.
If the stocks have a partial number, it is always the last digits. What would be the value of the first few digits, which would only match the stocks to a range of maybe several hundred thousand guns instead of to one gun?