Luis H:
Did you forget to put the saddle below the yoke nut (the curved fairing between the yoke and the main body) or did you intentionally leave it off?
Good eye. No, I didn't forget it, It's been cracked for a long time and it finally broke in two so I left it off. So what is the real purpose of the yoke saddle, looks?
I don't know if it is tight enough to provide any structural support/ backup to the yoke assembly. I think it will help prevent damage in the case that someone went to lift the tank by grabbing the regulator, I am not sure. That would be my first impression of its purpose, but I have to look at it closer.
I started repairing Scubapro MK-5's in 1971 (when I was working at my LDS) and know I am also a mechanical / structural engineer, but I have never actually given much though about the need of that part. I just thought that it just always belonged there.