The Plural of P1

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.