Steve,
I read in a 1939 RCA Review article about the TRK12, that they had to make the oscillator coil out of a 1-turn loop of brass strap soldered directly to the 6J5 socket terminals, and raise the filament voltage on the 6J5 to 6.5V, just to get it to oscillate at 100 MC. I'll bet there was some tube selection going on too.