The Retina is trapped! The selenium meter is coupled to the lens via a string, but is not attached to the body. When the upper hood is removed to access the cocking mechanism, the string jumps. And you have to remove the lens and its plate to put everything back in place. Anyway, I will have to disassemble this lens to lubricate the diaphragm and shutter speed rings, seized.
The culprit: a broken spring, behind the bottom cover; The one I put in replacement is not very straight, but it does the job.
Several reassemblies-disassemblies will be necessary to find the correct coupling between the meter and the lens.
Finished!