RFT was a larger concern than just the Gefell microphone factory, and even the Gefell microphone factory made a wide range of products, from strictly utilitarian (public address) equipment with no pretensions to higher quality on the one hand, to measurement microphones on the other end that set a very high standard.
Just FYI the Neumann (Berlin) U 47 replaced the original PVC membranes with tensilized Mylar (polyester) ones starting in 1959-60, and the U 67 had them from the beginning. Among manufacturers that are at all well known today, only Gefell continued to produce PVC diaphragms after that time. They also continued (and still continue) to make small condenser microphone capsules with nickel diaphragms, which Neumann, Schoeps and AKG abandoned as quickly as possible. Ironically, this means that some people have turned to Gefell to restore the capsules of their Neumann (Berlin) KM 54s since Neumann can't do that any more.
In the socialist countries it was sometimes possible to obtain certain things from the West by buying them through a neutral third country such as Austria, which had treaties with both "Germanies," or through another socialist country that had fewer (or simply different) trade restrictions. That's how the messy business of the so-called "Hungarian Schoeps" microphones began, for example. But that kind of trade was subject to uncertainties and a level of expense which could not always be spared. The East German government was fanatical in wanting to avoid any speculation or manipulation of their currency, or indeed any dependencies on the West to the extent possible, so imports and exports were tightly controlled as a rule.
Nowadays if you send an older Gefell microphone to Microtech Gefell for overhaul, they can generally improve its basic level of performance significantly. There are also individual technicians who have done a lot of custom upgrade work on older Gefell microphones. If I saw a 1950s or 1960s Gefell microphone that sounded good, worked well and had high-quality parts in it, I would assume that work had been done on it.
--best regards