Probably a pre-CBS Fender Jazz Bass.
I think the "holy grail" in the thread title might be misleading; a '59 Les Paul isn't really a holy grail because there are more than one of them out there. They may be rare, coveted, and iconic, but they're not unique in the grand scheme of things.
A Gibson Moderne on the other hand is the holy grail of guitars...so, what's the bass equivalent of that?
Fender Starcaster Bass prototype from the 70s?