That is really interesting. I thought the same thing - the XLR jack must be for powering active electronics. But the thing is, it doesn't look like there is anything that actually takes power. All of the electronics in the photos are passive. Based on what I can see, it looks like the controls are basically just regular "P-bass" controls x2, with a volume and RC-type tone control for each pickup. (The volume control appears to be just a 2-gang/stereo pot, so that it controls both pickups at the same time.) And then the switches are for things like phase and/or series/parallel per pickup, plus possibly phase and/or series/parallel for the combination of the 2 pickups.
This is a total guess, but the XLR jack might do something like enabling "stereo" wiring - i.e. separate outputs for each pickup, like the Rickenbacker Rick-O-Sound.
If you (or a tech) can trace out all of the wires and devices, and draw out a schematic, it probably would start to become pretty clear what each thing does.
In any case, the controls definitely appear to be an aftermarket mod! I'm not aware of any Fernandes FPM model (or the similar BO models) that had these controls as stock. Also, as you mentioned, the neck pickup position is more like the 1-pickup FPM-70 model. I really wonder if this was originally a 1-pickup model, and the bridge pickup was added later?
Whatever the case is, very cool bass!!
