Pretty sure it's stainless. Some are magnetic and some aren't, which you may already know. If you're really interested, here is a good article explaining a bit.
I just picked up a 2007 TTR230 header thinking I would polish it up like others have done but I noticed that it's magnetic. Does Yamaha use T409 SS that is magnetic or T304. Are there any other ways I can verify that this is really stainless?
'95 TW200G - TCI skid, ProTaper bars, amber Jimbo, DMO pegs, IRC GP-1, DG-O w/TTR230 header, Brisk BR12ZC, ProCycle jet kit, K&N filter, DID x-ring, TCI Sequoia rack, storage tube, JNS light
'04 Buell XB12S Lightning - Drummer exhaust, race ECM, K&N, open airbox, RC right side scoop, Corbin seat, fender eliminator Michelin Pilot Power 2CT