After a little more research it looks like the shock shares a lot of spare parts with the WP 4681 BAVP shock used on early LC4s and I think the early dukes. Turns out it's not a bladder design, but instead a floating piston keeping the nitrogen and oil separate.
Owners manual here:
...and a later version here:
Rebuild thread here:
Kind of an obscenely huge shock:
As far as rebound damping is concerned, "mine goes up to 11."