With only one exception, all of the PX-10s I've encountered had the classical Simplex steel seatpost. I LOVE those things, though, and still think it's one of the finest designs ever.
The exception was a bike with a straight seatpost and an alloy-railed Ideale 90 with the built-in clamp. Even that one had a steel seatpost.
Russ Fitzgerald
Greenwood, SC