When my Phon' was being designed over a period of time there was a few Circuits Produced of which a few were superseded. Even though the updated designs were a betterment ( Personal Preference as a result of listening evaluation). The earlier designs were introduced on occasion to measure where the differences being produced were able to be discerned for the impression of the End Sound being produced.
A Friend after being demo'd my Phon' on a few occasions wanted to have a model in their own system.
When having been demo'd an earlier Prototype, they remained smitten and after having it put into a Casing with a few component exchanges, had bought into the design at very very fair monies. My experiences of this Phon' is enjoyed as much as my own, there is only small differences to the voicing that I can detect in the different system.
There may be a Prototype/ Prototypes that still remains, which can be discussed with Mr Sachs. Such a Model may become after a little attention, a Bespoke Design that will due to Pedigree always be a Saleable Pre Amp, offering much of the attraction of the Amp' offered to the broader community?