Hi Paul; since you are in a hurry, I'll see if I can help since the Alembic staff are quite backed up at the moment.
The third character should be a letter and based on your description is probably an S, if you can check again to be sure.
If it's an '84 Spoiler, than the body is probably Mahogany. There should be a top body laminate as well; many from that period had Koa tops. If it's a Spoiler, then the three controls are as described
here.
There are pictures of Spoilers
. You can compare the bass you are looking at to these.
Generally, and roughly, on the used market a Spoiler in good condition goes for around $1,000 to $1,400 for a private sale. However, prices for used Alembics seem to be going up significantly recently, so my guess may be low. Dealer sales will be higher. However, UK prices for Alembics are generally much higher.
Hope that helps.