The valve spring cap has to screw up tight. It clamps the stem sleeve that bottoms against the step on the valve stem, the hat section at the bottom of this sleeve is what the fork of the rocker rests on. That is why it needs the locking top cap and small screws/nuts to stop the assembly from coming undone. It is similar in action to the post war set up, but more fiddly with more moving parts, that's why those small screws and nuts fall off from vibration and originally locked by securing with solder. That was quite a common method of hardware retention back in those days, you see it in magneto's and generators as well.