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The Burns website lists the current Marquee reissue model as a 34", was the vintage original a 32"? http://www.burnsguitars.com/marqueebass.php

My Hofner Club is 6lbs 10oz with a centre block so an Ignition is a good call being fully hollow.

Excellent!

It seems reasonable to suppose that white and red will be the hot wires, in which case yes you will be wired in series. Best thing is to give it a go and see how it sounds.

I've not used GHS contact core strings with a piezo but I have a tapered B string which works fine with my piezo bridge. The piezo picks up vibrations from the string and will work fine whether they're regular or tapered.

The white and red wires will be hot, black will be cold. Typically speaking you'd wire them in series by joining the red wire (hot) from one coil with the black wire (cold) from the other. White (hot) will then go to the vol pot and the remaining black (cold) to ground.

Hmm, that's a bit odd. Guitar parts with 6-32 thread are usually UNC to fit vintage Fender.

Thread size is UNC 6-32 - https://www.allparts.uk.com/products/bridge-saddle-height-screws-for-bass-or-t-guitar-slot-head-6-32-x-1-2-inch

+1 Looks like it has a vintage tint to me. The neck finish on my Musicmaster is much paler. Couple of pics below for comparison

https://shop.ehx.com/category/parts-knobs-electro-harmonix/

Yes a bit of googling provides an interesting read! It would appear that a single string can't produce anything lower than the fundamental, but a combination of two notes can produce a tone lower than the fundamental of either. This is called a 'difference tone'. A good example is the power chord on guitar (root + fifth). Because these frequencies have a ratio of 2:3, they generate a difference tone with half the frequency of the fundamental (ie. an octave lower). Apparently this is why power chords sound so fat.

A 55-01 with NTMB preamp sold on BC recently so may be worth having a chat with the seller

Pickups are passive (I believe the same as on the BB414) so no problem on that front with switching out the preamp.

That's very interesting, thanks for posting!

According to this thread the correct size is a rather tiny 0.05" )

If they're out of phase you'll get a drop in output when both pickups are full on. What's the volume like with just the neck pickup or just the bridge pickup? Having a low output doesn't necessarily indicate a problem (unless it's stupidly low). If you can adjust the input gain of your amp to compensate for the lower output and get enough volume then it's probably nothing to worry about. What model of bass BTW?

Nice looking bass. The pot codes may help with dating. Some other dating tips here - https://www.beatlebassdealer.com/dating-hofner-serial-numbers.htm