There are a ton of NOS preban P228/P229 9mm mags around. For those in the know, $37 is way too much money. You should be paying $30 or less apiece for them.
As far as the "how to tell" stuff, there is no conclusive line on Sig mags other than LE markings. (EG, obviously, the mags with LE markings are illegal) There are 900 threads on here if you want to see the arguments, though.
Generally speaking the older ones are phosphated (dull gray or black) and the newer ones are shiny... but this isn't consistent nor a reliable indicator. Zipper backs ARE older than zig zag backs, but both can be preban. (I have some of each).
-Mike