Just checked the bridge pickup, it clocks in at 7k.
Looking on Reverb, I found two pickups that look like mine and measure close to 1.2k like mine <snip>
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Given that you found those ads for mudbuckers that measure close to your pickup's resistance value, I would guess that Gibson produced a run of EB-3 basses with a high-impedance bridge pickup and a low-impedance mudbucker.
There were Les Paul guitars and basses in their lineup in the late '60s and early '70s that had low-impedance pickups (some or all of which had a built-in transformer to enable them to be plugged into conventional amps).
A further guess: Gibson intended the high-impedance bridge pickup in your bass to be used with a regular amp for live performances, reserving the mudbucker for studio work, where the bass was supposed to be plugged directly into a mic input on the mixing board. Maybe the rotary switch was meant to let the studio bassist go either way during a session.
While it's true that there's not a lot of room for a transformer in the wiring cavity in your bass, I'm pretty sure that you could cannibalize a $25 Shure (or equivalent) matching transformer adapter plug (XLR to 1/4 inch) for the tiny transformer element and wire it in.
Or, of course, you could pull the mudbucker and install an Artec mudbucker (around $35), which measures something around 30K, like the original mudbuckers.
More info, if anyone's interested:
The company also sold a companion amplifier, the LP12.
It was a monster, with a preamp head and a cabinet containing an amp that put out 150 watts RMS in a bi-amped arrangement that sent 75 watts to four 12" speakers and 75 watts to two 10" horns.
They weren't alone in going for the preamp head/powered cabinet arrangement. Fender had their Super Showman (see the video of the live Focus show where they play a hyper-speed version of their one hit, Hocus Pocus; the guitar player plays through a mammoth Super Showman), Sunn and Acoustic Control both had several such models, etc.
Gibson, unlike those other companies, didn't have much of a presence in the amp market. So when the LP12 sold poorly, after what must have been a big investment in design and manufacture, they pretty much gave up on selling amps.
(I bought an LP12 from Guitar Center for $150 about 10 years ago, mostly for the sheer magnificent goofiness of it. It's sitting in the next room right now. Just double-checked, and yes, the two channels have two inputs each, labeled IMPEDANCE HIGH LOW.)
Les Paul himself talked Gibson into branching out into low-impedance instruments. From a quick search:
- Les Paul's Preference: Les Paul hated distortion and preferred a clean tone for studio recording, leading him to design these pickups to achieve better clarity and overtones.
- Studio Focus: The instruments were originally intended for professional studio use, designed to plug directly into low-impedance inputs of mixing consoles.
- Reduced Noise and Increased Fidelity: Low-impedance coils have low DC resistance, which, while creating a weaker output signal, provides a much wider frequency response and cleaner sound.
