I think you've probably decoded as far as you can.
From everything I know - they are simply a sequential number used by body stamping
I would really have to examine in more detail some of the cowl tags from the years, 81 onward to see there was any hidden meanings in the numbers used.
I only have a small sampling of those years' cowl tags so not enough to base any opinions on.
P = Pontiac, you are correct sir.