Seems you're thinking too much inside the box!

A Road Worn five string P-bass would not have to be connected to any previous Fender P5 -- it would be for five string players (like me) who want a five string Fender with the mojo of a 50's or 60's
four string -- simple as that.
I once owned a beautifully mojo'd 1964 Fender Precision, 12/64 on the neck stamp. I now play five exclusively (so I sold the '64 to a friend), but I want similar mojo in a Fender-style five string. As it happens I'm having a local luthier build me one. The biggest challenge is finding five string parts that match the vintage four string counterparts (nickel-plated tuners and bridge, for example). Because of this mine won't be exact, but it'll be reasonably close, and I'm paying a small fraction of Fender CS prices.