No one cares about this except me. I think. That's OK.
The plural of "P1" is "P1's"; it is NOT "P1s."
Plurals of words ending in numbers are formed by adding 's.
Now -- what is the plural possessive of "P1"?
If one wanted to write about the passion of more than one P1, I would think it would go:
"the P1's' passion."
But I dunno.