Hi Simon,
Here's the detail about your bass from the file:
serial: 89AM061
name: 20th Anniversary Model
model: AM
top: Burl Walnut
accent: Vermilion
body: Walnut
back: Walnut
shape: 20th Anniversary
neck: 5-piece Maple and Purpleheart
scale: Not recorded!
fingerboard: Ebony
inlays: Pearloid (plastic) ovals
side markers: bronze
peghead: Crown
pickups: AXY
electronics: 20th Anniversary volume x2, filter x2, 2-position Q-switch x2, 4-position rotary pickup selector switch, stereo/mono switch
finish: clear gloss polyester
backplates: brass
hardware: polished brass
originally made for: The Bass Centre (not recorded if LA or London, but I'll guess London)
birthday: September 1, 1989
There is a curious note that says was Jason Newsted and I know he did get a 20th Anniversary model around that time. Maybe another one caught his eye? I dunno, but it is par tof the history, even if it's only a little bit of mystery.