When I first got my 51 Normale it did not have the interlock fitted. Whoever had last taken the top off it had just left all the bits out.
I haven’t tried to compare them but I would be surprised if a DS cover would fit a Traction box.
I cant quite get my head around the biting point being high and the cable being slack. If the cable is slack it should mean the biting point would be near the bottom (fully depressed) end of the pedal’s travel. That would mean your clutch might not be disengaging which would make changing gear difficult.
If the interlock balls are still in the top cover, but the operating rod is not there to move them, that would also make it hard to move the lever.
Chris