Another novelty! For the troubleshooting, it comes close to the Minolta Hi-Matic and the Konica Auto S2: you have ti find how to remove the lens.
There it's done!
Behind the lens is the speed control mechanism, rotating toothed rings with a return spring. It's stuck. A bit of penetrating oil, I make work, I lubricate slightly, remains to reassemble and to control. Everything is working!
In parallel, my customer sent me two Zenit 3M to review (and their lenses for grease replacement).
Recurring problem on the cameras built at KMZ around 1960: the axes of the shutter were lubricated with a grease that has deteriorated. This grease must be dissolved with a solvent deposited with a syringe and the shutter will return to normal operation. At reassembly, of course, I re-grease the film advance mechanism.
The small family ready to leave.