I read in the interview John assembled the bass himself, soldering iron in hand. I have no idea if this is correct. So I wonder was one pickup from the carcase basses just a bit hotter? Or maybe a capacitor of a different value? Maybe it was a odd pairing of potentiometers?
John Kallas told me that the Slab circuitry was bone stock, as per regular P basses. He got to explore an original one. Likewise the one I've seen dissected online had normal Precision circuitry, though two different coloured bobbins were used on the pickups; a real mutt.
My unproven theory was that Jazzmaster 1 M ohm pots were used, because the original request had been made for a 'Telecaster bass' from Fender, and the first run of slabs was a response to that. 1 M pots would be brighter, and more Tele-like, and were a standard Fender component in 1966, though I have no evidence to back this up and evidence that refutes this idea! The classic Leeds tone doesn't have any perceptible treble roll-off, so surely the cap would be less of an issue?
Gareth, who is behind the Slab 66 basses, builds them with a hotter pickup apparently. I've quizzed him and he doesn't do anything else deliberately to get slab tone. Conversely I've got a pretty good Slab tone out of a Yamaha BB bass with low action, fresh rounds and a tube amp emulation.