Hi, Derek,
The guitar sound on The Magical Mrs Clamps - especially on the chorus played on the lowest strings - is pure Burns Marvin (original model). The main component is not the pickups, which, as others have said, aren't all that different from Fender pickups.The main contributors to that particular sound are:
(a) the Rez-O-Tube tailpiece, which imparts a depth on the lower strings which a Stratocaster simply cannot match, and
(b) medium gauge guitar strings - almost certainly Gibson Sonomatics, gauged at 13 / 17 / 26(w) / 36 / 46 / 56.
My 1965 Marvin (bought s/h in 1972) nails that and similar recordings (eg, the bass string middle eight on the instro version of Friends) absolutely effortlessly (fitted with appropriate strings). I have several Stratocasters, including two USA models, and I don't think I'd ever be able to get that "Burns low string" sound from any of them. The strings simply ring differently.
Mind you, the recent flurry of reissue Marvins don't quite catch that original Burns sound. This difference has been put down to the newer Marvins (Asian-built models) being voiced more like Fenders (which I assume means that the pickups are underwound as compared to the originals or to those on UK-built Burns Legends).
HTH,
JN