We really need to see a few photos. This is to judge condition as well as see what sort of stamp it has. Not all stamps which have a moon and star with rays are Formula 602. But I can't tell from your description.
There are at least three types of vintage 602 stamps. You can see these via the link in my signature. Here are other options.
Not a Formula 602:
[img]http://black.net.nz/paiste/images/Paiste-Giant-Beat.png[/img]
[img]http://black.net.nz/paiste/images/Paiste-Ludwig-Standard.png[/img]
versus a Formula 602 which doesn't say Formula 602 on it
[img]http://black.net.nz/paiste/images/Paiste-Mixed-sans-602.png[/img]
Once I know what it is I might be able to give you a more accurate price, and possibly a production era. A weight would also be useful. But even if we get a positive id and it is in great condition don't get your hopes up that it is worth more than $80 tops. Unless it turns out to be something special and unusual.