Well - this is interesting.
Firstly, 'Custom' was a Netherlands importer brand, I think for a specific retailer - somebody on one of the Facebook MIJ guitars groups found the name but it's slipped my mind, annoyingly. They sourced from a number of different manufacturers including Moridaira, Matsumoku & Chushin - so they probably just bought in whatever their supplier had, rather than contracting to specific manufacturers.
Customs turn up in the UK pretty regularly, very often in Scotland, which has led to some speculation a Scottish retailer was bringing excess stock over from the Netherlands - no evidence as yet but no-one would be surprised if it turned out to be the very entrepreneurial Jimmy Grant, who had Glasgow & Edinburgh Grant Music shops, his own range of Grant guitars (with UK distribution) & was also UK importer/distributor for Canadian hand-made Odyssey guitars & basses in the 70s & 80s.
As for the bass itself - that's really interesting - and this is why:
Had this back in 2011, and I could never pin down what it was. This had a plain, oddly home-made looking neckplate & the body was a not-quite right P shape. Came with no hardware, and I pretty much concluded it was likely European, not MIJ, based on the Eko-esque inlay patterns. I have never seen the same neck before today. I think the finish had been stripped under the rattlecan black & it had no hardware or electronics to help ID it - but seeing @Shabbs's pics I'm confident it's the same bass. No logo on mine, the head having been stripped & re-finished, probably with Ronseal!
I'll speculate & say this might have come from an organisation called Matsumoto Gakki Seizou Kumiai, the Matsumoto City instrument manufacturers' co-operative. They bought in bodies, necks, electronics & parts from various small manufacturers & assembled instruments from an order-book system, where a customer would choose necks, bodies & parts, and instruments would be built up & branded to their specifications.