Hello Terry,
Having had a scout around this afternoon it would seem Marcus’s bass is an early Sadowsky 5 made sometime mid to late 1980’s. These versions had 21 frets, electronics wise they were fitted with Bartolini pickups and circuit. I think you’re spot on with ref to the headstock config, the 1990 Jazz bass plus which was the first modern 5 string bass from Fender had 5 inline tuners, as opposed to Sadowsky’s 4+1.