That's actually "PI 438638".

And no, I don't know where the number came from, but there are Google hits on the PI number.

It's probably PI, not P1, fwiw...that would be a USDA seed bank naming convention (along with GRIF).

Various seed banks/registers have their own naming conventions...mostly based on their storage key/number.

To add to the confusion, some hobby breeders have their own naming conventions based on what plant (out of 100s/1000s) they selected the seed from and it's characteristics (H = hot, F = more flavorful, etc). So...that P1 may actually be a P1, not a PI.

There is no universal naming scheme.

PI , Grif #'s aren't dictated by any growers.
It's the Gov. classification system.

CO is another along with CGN,CAP etc.

http://www.cgn.wur.nl/applications/cgngenis/default.aspx

Here is a link that might be useful: Gov. data base for PI #'s etc.