If in the former eastern countries (Comecon) a lens constructed for medium format, would be in Kiev 88 or PentaconSix /Kiev 60 mount. There are adaptors from Kiev 88 to Kiev 60 mount, which ist identical to the Pentacon Six mount (P6).
P6 is good for us, because the flange distance (74.1mm) fit to Pentax 645 and there are various adaptors P6-> Ptx645 on market.
I looked in my literature for medium format east mirror lenses (Scheibel, Pincelle, Pietsch). No russian mirror lens is mentioned*, only the east german Practicar 5.6/1000mm. This "Prakticar" wasn´t manufactured at FOG (VEB Feinoptische Werke Görlitz [former Meyer, Görlitz]) as the name "Prakticar" would suggest, but at VEB Carl Zeiss Jena. It seems to be rare. My suggestion is it wasn´t sold often to the public ( orignal price was something over 10 000 M**), more to state organisations (army, secret service, police....)
Pentacon Six: 1000mm lenses
* all russian mirror lenses seemed to be M39/M42, but may be there are "after manufacture" adaptions to P6
** something above the price of the east german "Trabant" car.