EARLY 066 and 064 tanks are interchangeable, later 066 and all the MS650, MS660 use a different tank.
There is not an 064 aftermarket tank so the 064 tank is worth more used (in general) than a used 660 tank.
$65.00 is a very nice price for a decent 064 tank.
I recently bought a New Old Stock 064 tank without any of the bits n pieces for $200.00
660 tank is wider and holds more fuel than the 064 tank
Dave