They're not for real heavy hitters. Even though 602 hi-hats were used by some hard rockers, like Bonham in his pre-2002 days, 602s were meant to be mellow and crystal-like cymbals for studio, jazz and softer music in general. The overall spectrum of frequency was narrow compared to say 2002s, which have a broader frequency range and more volume. So basically the original purpose of 602s was to not overpower acoustic music, so they have a relatively clean sound without many overtones. Paiste beefed up the 602s in the 1980s and came out with Extra-Heavy hi-hats for heavier hitters (Edit: Actually- "Extra Heavy" 602 hats were around in the 70s too). But most of the 602s were not meant to be bashed hard since they are somewhat thin and some drummers would overplay them to get the volume they desired, which the 602s were not really designed to do.
I think the Sound Edge 602 came out in 1967.They were the first ever sound edge hats ever produced.The very first sound edge models were extremely thin and had many ripples compared to newer models which had fewer ripples but more weight. Many of the old 602 sound edge hats have cracks in them. Check them out thoroughly to see if there are any cracks, if they are indeed sound edge hats.