Right, so... The first issue with trying to swap these older pieces into your shorty are:
#1 your hum canceller is glued in place and not easy to swap out without literally using a hammer. Sometimes they have to drill into it and use a giant screw to pull it out (or maybe I dreamt that, but I'm pretty sure about the hammer). I kinda doubt you want to go that far to make an A to B comparison.
#2, take a close look at the size of that "spare" hum coil and compare it to the one in your bass now. There's a good chance it is NOT exactly the same. And there's no way to know if it's the same depth.
#3, the connectors. I lived through the transition period and actually have two (oddly, not three) adaptors that I got from Alembic way back when. See the pic attached. The tiny coax connectors on the pickup side are highly unusual (in audio) and I'd be surprised if your local repair guy had ever seen one - unless he also dabbles in microwave work. Furthermore, these adaptors are to get from older model pickups to a newer model preamp card, not the other way around. So assuming you don't want to bust out your hum coil, I don't know if there was ever an adaptor made to connect new pickups to an old card. That could be rigged but you'd need to make a solder connection directly to the board.
So ... if you had these two adaptors that I'm looking at (and Mica may still have some) you could swap in the older pickups (assuming THEY fit) and connect them to your current preamp card to see what happens. However ... does your bass have Ron's magical noise-reduction mod? If so, those fine-tuned components will no longer do their job properly if you swap pickups or hum canceller...
I don't mean to throw water on your plans but I must also add that I believe any difference in TONE between old/new pickups, and old/new preamp boards, will be extremely subtle and very difficult to notice. Alembic has never "cut corners" on components. It's never about cost. They only make changes if a component is no longer manufactured, or no longer made to the specs that they require. There is no magic "pre-CBS", "best vintage" situation with Alembic, if you see what I mean.
Jimmy J
In respect to 1 & 2, I don't really want to remove the hum canceller if it is going to be a destructive process for risk of damage to the bass and also that they may not be the same size as you now suggest could be a possibility.
3, the electronics guy I am thinking of is someone who repairs radio and TV equipment both vintage and modern day stuff so there is the possibility he will know where to get them even if he doesn't have them himself.
I do have a mate (also a bass player) who has a business installing and setting up Cell phone networks and repairing RF equipment for Taxi's and security companies so hhe may be able to get hold of some.
My shorty does not have the noise reduction system and I believe these pickups in it are now slightly microphonic although they still sound great. As for the difference in new and vintage pickups, i'm sure you are right though it would be good to know that these new bits actually are useable in the event of me needing to utilise them at some point. I will contact Mica to see if I can order some connectors if I can't get any made locally. Thanks again for your input. :-)