It's really very hard to find and back in the day, I ended up asking my friend Christophe Leduc to build me one sixer EUB, a totally unique piece.
It only took 7, yes seven, years to get it...
Had a few Jazz Basses from Signature something like 15 to 20 years ago.
Excellent stuff for sure and indeed made in Canada as a collaboration between them and Alex Lifeson.
If it helps, the last retail price on the 1st of August 2019 for a Maurad 4 (the bass this one was the precursor to) was ... €4900 Euros, so £4135 GBP at todays exchange rate: At £1149 GBP is even more than an utter bargain!
Try lightly sanding the top of the G saddle, it could be a very slight connectivity issue.
Or maybe the silk of that string is on the saddle preventing good connectivity (grounding).
I see that your shielding is not grounded...
Ground it as well as the pickup cavity and shield the back of the lid of the electronics compartment that will be grounded by contact with the cavity shielding.
Did you earth ALL shielded cavities including the pickup one?
If not, you have the culprit, because if you shield something without grounding it's totally useless (and way too common).
And did you shield and ground the back of your pickguard as it plays a role in protecting the whole electronics from electromagnetic fields?
Check this before saying that all pots are equal: https://www.potentiometers.com/potcomFAQ.cfm?FAQID=19
CTS aren't better than other carbon track potentiometers, they are as bad and cheaply made.
It's an urban legend based on vintage collectors bad appreciation.