It is a mild steel, used for a while until it too ran into bending/breaking problems.
Brian had told me he had intended to do the latest ones in brass, though I'm not sure that is the case anymore (mostly because of cost I believe, though I told him the brass would be much better than the aluminium he had been using, regardless of series)
EDIT: To add, I think that the brass would be stronger, while being easier to machine than SS. I could see where anodizing the current designs would help tolerances, albeit the fact that the anodizer they use tends to have anno that's on the thin side (just look at all the dust black Gen1's, heck my feed tube got scratched just from being put on my desk.)
And to throw my hat into the ring, I do like the current version of the piston, I still use the "thinner" one as of right now with no issues. Taking the "set it and forget it" approach has worked out for me so far