The Josephson 617 uses standard the Microtech-Gefell MK221 1/2 inch capsule, which is the equivalent of the discontinued B&K 4165. In fact those capsules are successors of the 1 inch version 4145. B&K has discontinued the production of 1 inch measuring microphones, although they might have some on stock.
Problem with the 50mV 1/2 capsules is, the lower noisefloor can only be achieved by using some mechanical acoustical tricks, what one has to accept is a rather low bandwidth for a 1/2 inch capsule (only 20kHz max), and you pay it with unpredictable behaviour above 20 Khz, there are some very strange bumps and dips before the final roll off.
In a way the 1 inch and 1/2 inch 50mV types are quite alike, and using them for music recording is a bit questionable. The main difference between them is that the 1/2 inch version stays omni directional for much higher frequencies.
Both Sonodore and Josephson use the limited bandwith 1/2 inch versions, Josephson uses Gefell MK221 and Sonodore is a bit mystical where they get the capsules from, but I think they use ACO Pacific titanium diaphragms.
Both designers probably know the phase characteristic is not that smooth as with the low output 4133 12.5 mV type of 1/2 inch capsule, but probably there is a demand from the market to produce these models.
It is no coincidence B&K (DPA) did (re)design one of their measuring microphone concepts to the 4006(3) model in the early eighties. If the phase issue was not serious, they would have never done that. Designing new models costs a lot of research money.
I use my 4145 1 inch capsules for very specific situations, and if I could decide today again, I would not buy them. I use them only very few, and it is not worth the investment.
Even when you try-out a microphone of this type, you may have other opinions some day in future. To sell off these odd 1 inch models may take a while.
Erik Sikkema