The first letter is the manufactoring site.
P = Inventec (Pudong) Corporation
The last letter if present is the hardware revision.
No letter means it's the 1st released version.
A = 1st hardware revision = 2nd hardware version
B = 2nd hardware revision = 3rd hardware version
... and so on.
Do you have pictures of the other side of the main boards ?
It's usually interesting because that side does have the board reference and the memory chips (SDRAM, NAND and on some revisions NOR).