They look like John Birch mini humbuckers, I just couldn't see the bajillion pole pieces that JB pickups tend to have. It could be down to image degradation or the film it was originally captured on.
Slade had a close working relationship with John Birch, so I wouldn't be surprised if the bass made multiple trips to him. When the bass was refinished white it also lost the mudbucker and gained a bigger batwing pickguard. After the mods the bass was closer to the custom basses John Birch built for Jim, so maybe Jim requested the mudbucker be removed and it was the refinish that was above and beyond the work requested.
It's a John Birch "Superflux" pickup. It only has single row of nine poles. They were John's attempts at a "blade" style pickup. They were very overwound and had a lot of range. It also meant that they could be used in guitar or bass, so Noddy Holder has one too in his Telecaster neck position.
This is my Tele with one in it.
The refinish of that bass happened a couple of years after the above video. I met and knew John after he built me a bass and he told me that the mudbucker was disconnected. Jim mentioned in passing to John he was "thinking" of getting a white bass. The next time his bass went in for a set-up John took his cue to refinish and restyle it. The mudbucker was removed which made no difference to the sound since it didn't work anyway.
Jim was not a happy lad, livid in fact, but he didn't have it for long. It went back into John Birch's workshop over Xmas of '75 and the place was broken into over the holidays, the bass was stolen and never seen again. It's out there somewhere.
