You stumped me. I haven't seen a P1. How about posting a picture?
Three different decoders also came up unknown code. If your tag is corroded you may be reading R1 as P1.
Not all letters of the alphabet appear on row 2 of 68 tags. If you counted letters thinking they are all there, (the I and O are not), then a rusted out tag could understandably be misread as P1 not R1.
R1 is the Music Master Radio, 2 watts of power!
Picture please.