via this auction
"Kurzweil K150 Fourier Synth Rack from 1986. If you are not familiar with the K150, it was Kurzweil’s only module to use additive synthesis, much like the Sequential Circuits VS [the Prophet VS was not additive - see Robbie's notes in the comments]. It comes loaded with 69 presets sounds in memory so you won’t have to start from scratch. It also comes with the FS-SMP Fourier Synthesis modeling program written by Hal Chamberlin."
I believe Wendy Carlos also used one of these, and I'd heard she got a IIe specifically to run the app. She also, if I recall correctly, designed some of those alternate tunings.
For those interested, several of the tracks on my second, unreleased Periodic Music album called "Walking Slowly Backwards" feature the FS150, in particular, a piece called "The Naga" in which the lead voice is the 150 controlled with a Lync LN-4, using (I think) one of Carlos' alternate tunings.
You can listen to those tracks here on my site:
http://www.otownmedia.com/he/walkingslowlybackwards.htm"