Very rare, but at least 3 are known to exist. In the 70's Portuguese band Tantra mentioned one on their album covers.
Terrence O'Brien about his TG-55 Synthesiser
I am he original owner and purchased the TG-55 from Kalhua Music in Toronto circa 1976-77. I had been asking about a Synthi Hi-Fli and one of the staff told me about the TG-55 having just seen it during a recent trip to the U.K., so they ordered it from England once I had expressed interest - not sure if from Top Gear directly or not. This unit appears to be the first one made.
"No.1" is written on the inside chassis - Serial # 508-1.
For many years I thought it was the only one. I then discovered one at a studio in France (Sphere Studio) then about two years ago came across a picture of one being used by 808 State and owned by Graham Massey - who I've had contact with, but neither know anything about the units history. I also discovered a late 70's prog band from Portugal called Tantra - their guitarist used one as listed on two of their albums at that time.