Surely the pickups would be easy to replicate though? Alleva Coppollo and Moollon are supposed to sound close but from clips I’ve heard it’s not been right to my ears. But then some of the upgraded pickups in a vintage bass, such as early EMGs still sound vintage to me.
My thinking is it was the wood source. These days a lot of the wood used for guitars is from sustainable forests in Asia where the trees aren’t as old as now protected forests in US, South America, or Canada. Where was the wood Fender used from in the early days, is it still available? The difference in climates and age will affect the density of the wood.