It's delrin. Would be a little heavier than the original, being a bit bigger, but it is slightly hollow so wouldn't be much in it.
One of the o-rings is the main sealing seal (top one), the bottom groove is slightly smaller so that o-ring is mostly just guiding/keeping the piston square.
I do have some NBR u-cup seals on order that I want to try on a different IFP. It will be similar (to this one), but with the u-cups opposing each other. Upper one sealing the chamber, lower one sealing the positive air spring side.