Hi thanks for your interest in this rare beauty, more info can be found here [url="http://www.alembic.com/info/fc_spyders.html"]http://www.alembic.c...fc_spyders.html[/url]
They say they were planning to make 25 of the 8 strings however I believe they only ended up making 20, the build number and details above are from the serial number search from Alembic. It does have 20th Anniversary electronics.
This bass weighs around 11lbs (~5kg),however it is extremely well balanced, if you see pics of John with this bass, it sits very nice, no dive from either end, similar to his Buzzard design, both basses sit and play very comfortably (of course you need to have a nice wide padded strap) . The high wing design on the Spyder and Buzzard also serve as a great arm rest and help with the balance. These are also very long basses.
As far as the build quality from Alembic, it is just flawless, pure craftsmanship and art, this thing plays better than it looks and the sound has to be heard to be believed.