I could be wrong!
This bass looks lovely and went googling to find out what the Bart 9W5 and 9W4 pickups were, apparently Bartolini humbuckers with a coil per string... so five coils per pickup....Roger decribes them as linear hum buckers... I dunno, could be wrong. but low output so the black box is a bartolini booster before the Sadowsky preamp on the PCB https://www.talkbass.com/threads/club-sadowsky.390234/page-81#post-5800314 I don't know what that all means in practice but would love to try this one to find out!