> I doubt they can show a properly registered copyright, which would have been required before 1978. I doubt the copyright laws back then would have even allowed copyrighting the shape like that (but I'm not a lawyer).

None of that matters to Fender's case here, though. They benefit regardless of the outcome in court. If someone fights them and Fender wins, or no one fights them, then they cause an enormous, permanent headache to almost every single one of their competitors. If someone fights them and Fender loses, they cause an enormous, temporary headache to almost every single one of their competitors and otherwise there's no change in the market. The worst case for Fender is the status quo, there's no reason for them not to pursue this.

The only way Fender loses here is if they piss off enough customers to cause a drop in sales. But that seems unlikely to me, even extremely pissed off customers forget about these things pretty quick, as Reddit and Elon Musk's white supremacist social network demonstrated after shitting all over their own users and seeing no terribly significant drop in usage.