I'm skeptical
Yes, but I can't explain the tailpiece otherwise. It looks stock and it's a 5-string unit.
Also, all the tuners look identically distressed (whatever was left of the gold plating) and look the same as my '64 single-pickup version 6070.
Those necks were so wide and thick, they could easily take the pull of 5 and even 8 strings at full tension. The headstock is enormous and can take a 5th tuner, but the resulting neck-dive - a major problem with the 4-strings - would then make the bass impossible to play standing up, unless you add weights to the bottom strap button.
Looks like previous owners have done the same mods that I did to mine - relocated strap button to handle the neck dive, and painted-on side dots.
Mine has a spike to be played like a cello, that goes through the tailpiece hinge, this one doesn't.
Also, where would you find extra-long 5-string bass strings in 1968? These basses need longer strings than a Fender's, because of the afterlength. When standard long-scale strings are used, the silk goes over the nut almost to the 1st fret.
My gut feeling - and this is not supported by undisputed facts - is that this could be a factory mod, but the result was unplayable, and so the experiment never evolved into a production model.