I just found a very interesting paper - a complete list of all transmitting amateur radio stations in 1926 in the USSR, club stations, as well as personal ones. It is remarkable, that the frequencies and power levels of club stations are selected in such a way that ensures reliable traffic between distant regions and Moscow - the farther station from center, the higher its power and shorter its wavelength. And another note - individuals had managed to use different wavelengths, so it was not too easy to contact each other that times.
These power pentodes has been used as a transceiver final (one or pair of ones in parallel), in a modulator for the more powerful tube in AM transmitters, in a driver stages and (when SSB times comes) in the most common general class amplifier - three (or four) GU50 in parallel with all grids connected to the ground.
GU-50 is russian version of WWII Luftwaffe LS 50 general purpose HF power pentode by Telefunken, also known as RL 12 P 50 tube in Wermacht's transmitters. Main difference between russian and german versions - socket construction and dimentions, so I'm not quite sure that it is possible to use russian GU-50 as a spare part for a vintage german transmitter.
There are several other members in this family, such as DDR SRS 552 M tube, SL 152 (~12.6V filament) and EL 152 (~6.3V), but I don't know any more about these tubes, only that these tubes exists.