They're pretty rare, but not nearly as rare as dividers for the 70. Hopefully you got the rest of the plates with it, as those by themselves are really really rare and certainly cost a bundle from Schaublin. There are a few variations, mostly to do with the driven gear; some are fiber, some (like yours) are properly made to mate with a worm, and the most recent ones I've seen have the driven gear with teeth cut at an angle to match the helix of the worm but straight cut. I like the fiber personally as you can give it a sort of preload with the worm. On some the plunger will lock in the 'up' position when rotated 90 degrees, which is kind of nice.
The driven gear has a separate hub, there were hubs available to match w20 or w25 spindles.
Complete sets seem to go for 6-800 bucks.