Dear 33d6 and dear R, thank ou so much for your help.
I took some photos, and as you can see, the cluch lever and the gear changer mechanism feet very well in the gearbox. They could be from other bike, but they look made to this gearbox.
Looking at the patina, they look very very old.
33d6, what parts from my images do you think that don't bellong to a SA gearbox? Just the cluch and gear changer mechanism or you also include the inside parts?
Another strange fact it's the location of the bike serial number. It's on the frame neck, not under the seat.