Originally Posted by rylan

There are no true P/A/G fare basis that I am aware of, with the one single exception being the odd A basis for the LAX-SFO shuttle when it started.

All P fares are comprised of a fare basis (generally with a /WNUP designator after the fare) that books into the P bucket. There are also two types of UP fares that book into A... the /WNUP, and also the old school xxxxUPxx fare that actually says the basis is first if you look at the rules.
Note that the /WNUP basis also costs, sometimes substantially, more than the basic coach basis. So don't let agents tell you that you only have a K coach ticket... its likely that /WNUP that booked into A cost you a few hundred more than the straight coach K basis without the /WNUP.

Anyway, if you buy a P/A/G ticket, its a first class ticket.