Heh......Not only have I seen this before, but he was following my directions! The owner of this AEB-1 called me a few weeks ago. He's a music store owner and a Luthier, and he was able to buy this AEB-1 for a very low price. I think he said he got it from a pawn shop, and it had come from an estate sale. It's in really nice condition overall, all original, original paint, complete with all the hardware. The only thing wrong with it was that it had no sound output. That's why it was being sold so cheap. He bought it right away, assuming that it was something that he could fix......
When he opened it up, he discovered that it was missing the Mystery pickup coil assembly! Gone, removed, stolen! The diaphragm was there, and everything else. And the bass was all assembled and strung up and played just fine acoustically. A weird situation. My guess is that sometime way back, the pickup coil had failed. The owner took the coil out and started the search for a replacement..... And put the poor Ampeg all back together and in its case in a closet. And it sat there for decades......
So the current owner found me, of course, looking for help. I've made up replacement Mystery pickup coils in the past, including a few of my Super versions. But right now, I can not take the time to stop and make him one. Too much other work backed up.......
But I told him how I repaired several AEB-1's years back before I started making my own pickup coils. I found that a Seymour Duncan Quarter Pounder P-Bass coil set would work just fine as a replacement coil in a Mystery pickup. As long as it was mounted and adjusted correctly, the sound and output level was almost exactly like the original. I repaired several "dead" AEB's like that in the early 2000's, with SDQP coils inside.
He happened to be a Seymour Duncan dealer, and had them in stock. I talked him through how to install them and set them up. And that's what he's done. I also talked him through the trick of adding the extra capacitor.
I guess he's putting it up for sale on Reverb now. Assuming that he's got it all working right, it should be a really nice Ampeg for someone.
The picture shows the coils down in the cavity, with the diaphragm removed. The coils detect the motion of the diaphragm, not the strings. That's the Mystery of the Mystery pickup.