Stainless -- and anyone else that looks for the date:
First of all, I don't know if all helmets have a "manufacture" date stamped anywhere. A few brands do -- I haven't seen it in very many.
If you're asking about the Snell Foundation sticker, look carefully under the lining of the helmet. You might end up having to pull some of the lining away from the padding, and that lining might be glued to the padding. The sticker is about 3/4 inch by 2 1/2 or so, and is printed on a reflective foil (that'll be damaged if you try to remove it). The numbers (i.e. M95, M2000, SA2005, etc) are often hard to see because of the reflective background and the faint ink used to print those numbers -- but they're there. It's very common for us to hear, when we're inspecting the helmet, a racer say "I've never seen those numbers before this."
Dig deep. They're not even placed in the same relative location from one manufacturer to the next. On some helmets they're found at the back of the head, some by the right cheek, and so on. But if it's Snell Foundation rated -- there WILL be a sticker in there.