Hi Lee; welcome to the board!
Yea, that knob kinda sticks out on a bass that the listing said had not been modified in any way, hee hee. Hopefully it's just the knob and not the pot that's been replaced. You can order a new knob from Alembic.
Nice looking Spoiler though!
An '86 that hasn't been modified would have AXY pickups. Fatboys can be ordered from Alembic and are a direct replacement. Both pickups are great pickups; the Fatboys have a little more growl, while the AXYs are cleaner.
I've been watching Alembic prices fairly regularly for at least the last seven years. Prices of $600 and $750 are not representative of the market during that time. Those prices are either from the last century or represent purchasers from sellers who didn't know what the bass was worth. Bob's suggestion of $1,500 to $2,000 for the US market seems like a reasonable estimate; and I think you can probably rely on Flip's (fc_spoiler) estimates for the UK market. I sold my Spoiler two years ago for $1,500 here in the US, which was the market price at that time. Since then, despite the downturn in the economy, market prices for used Alembics have continued to go up.