Sequential's C64 thingy

[Hallgeir Helland]

Hello all DIYers,

I recently got a copy of Vintage Synthesizers by Mark Vail
(great book, BTW), and I found mentioned an interesting 
device by Sequential Circuits.
 It is not described in detail, nor have I found a name, but it
is (or was) an add-on keyboard (with software) for the Commodore
C-64. It seemed rather interesting to me.

So: Does anyone have any info on this thing? Or know where 
to get info on it? Anything at all would be greatly appreciated.

Thanks,
Hallgeir

BTW: I never knew Stopp had worked on Emerson's Mt.MOOG.