They call it 'gas pipe' tubing in the description but in others they say it's basically Reynolds 520. For me gas pipe means 'hi ten' or 'plain gauge' i.e. the most basic steel you can make bikes out of, which is what my delivery/town bike is made from. No butting, no fancy heat treating, nowt.
520 is 525 that is made by a manufacturer other than Reynolds, it wouldn't mean much with the sticker, it means absolutely nothing without. Nowhere in the PX description does it say it's butted or heat treated, so I'm going to assume it isn't. Then maybe it is, marketing again, people will buy without knowing.
At least we can be pretty sure these are PX frames, unlike the new Ti range which is straight from the Alpkit catalogue, about which they're not happy, some of the early preview shots even had the wrong logo.