I think Nagaoka is the manufacturer for the headshell you are talking about. I just opened up a Nagaoka MP-50 today, and reading their documentation was interesting. They are an OEM manufacturer as well for 'other' companies providing various components and such for cartridges. Further reading (so rare to find a cartridge with any substantial documentation no?) the MP-50 comes in another version called the MP-50H, the 'H' being the supplied headshell for this cartridge. The diagram showing the headshell is the same as the Sumiko/ZuPreme in question.
So there is one possible answer to your theory of Sumiko and ZuPreme sourcing from a 3rd party - Nagaoka. It was Raul's posting on MM cartridges that motivated me to try a Nagaoka, I had no idea what I'm in for, but I'm sure it will be good.
So there is one possible answer to your theory of Sumiko and ZuPreme sourcing from a 3rd party - Nagaoka. It was Raul's posting on MM cartridges that motivated me to try a Nagaoka, I had no idea what I'm in for, but I'm sure it will be good.