Lol.. this thread turned into Material Science 101. I'll just throw one more stinker on the pile:
316 is the "better" stainless for avoiding corrosion in marine environments. However, when it's cold-worked it can become magnetic (I don't know - is this because the cold work aligns domains? Or somehow changes the lattice, losing it's 'austenicity'?)
Anyway, the point: 316 cold-worked into a screw may become more magnetic than 304 (or other flavors) which have seen less work. The 'less magnetic is better' test would reject cold-worked 316 stainless.