The early cases are definitely a rare find. It's probably because just about all of them are still with the basses that they came with, or people have snatched them up to go with said early basses.
I'm just happy that both of mine have cases that are made for them, and even happier that I was able to get the case candy (key and original allen wrench for neck tilt) from the one that I rescued from the pawn shop.