In the US, I'm pretty sure they have no protection. They lost a US case to establish a trademark in 2009 [1] (2009 article title says copyright, but text is about a trademark suit). A design patent would have expired, unless it was filed in 1954 and was pending until recently or is still pending... US patents filed before 1995 don't start their validity period until issuance, but that'd be a big stretch.

It's possible there's a US copyright claim, but on a 1954 design, you would have to have registered it, marked the works with the copyright (on at least most of the copies), and timely renewed. There's also, IMHO, a solid question of if US copyright applies to the shape of a guitar. If they had a strong case, I think they would have tried to enforce on it in 2009 when they tried to enforce on trademark.

[1] https://www.musicradar.com/news/guitars/fender-loses-guitar-...