re: the decade by serial: that's always been the case. But the weights; stamps; patina; lathing and general appearance let you know. This is indeed more difficult with highly polished Paiste cymbals. I can tell by many means; including the smell and the taste. I kid you not.
Since I've collected most of my info over the internet I've never had the chance to do much with smell and taste. I'm impressed. Cool1
When you have a 6 digit serial number on a 602 which starts with 4, then from what info I've got (and the wiki)
http://www.paiste-only.com/paistewiki/index.php?title=Serial_Numbers
It will very likely have PAISTE or PAISTE 602 right above the serial number if it is 1984. You can see some of these in the examples in the wiki, and they appear from 1982 onwards. I say "very likely" because in all factories (however well run) there seems to be room for 1 in 10,000 or so cymbals to come out with missing stamps, double stamps, etc.
But pricewise, if you have a Blue Label 602 from 1984 with every trace of the Blue Ink labels polished off it won't attract the price premium that cymbals from the Blue Label era usually get. If it was a Blue Label 1984 which mysteriously doesn't conform to the standard serial number schema, then whoever did that likely polished $100 of the value.
I thought about asking if you had a PAISTE or PAISTE 602 (or any other text) over the serial number but I figured you would have said, and I kept my already large volume of information a little shorter. Now you get more of the story. ;)
And apparently more of the story than the person you emailed at Paiste has at his fingertips. Paiste (and Zildjian, et al) spend their time on researching new cymbals and manufacturing cymbals, so I don't begrudge them not spending as much time on the history as we Cymbalholics do. I'm much happier for them to keep making new ones than pour over the history. And we just concentrate on being data detectives. I don't have to spend time making cymbals or running a factory.