This category is for synths that were built but never went into production (examples: the ARP Centuar and Moog SL-8) or were produce in extremely small numbers (example: the Con Brio ADS-200).
“A pioneering electronic music instrument, created by the RCA corporation in the 1950s. There...RCA Synthesizer
“A very large modular synthesizer originally assembled by Malcom Cecil and Robert Margouleff...T.O.N.T.O.
“A legendary ARP Instruments prototype, which the company built and rebuilt several times between...Centaur
“A prototype synth developed by PPG in the 1986-1988 timeframe, and most notable for developing...Realizer
“Generally thought to be the first electronic music instrument, the first Telharmonium was built...Telharmonium