Always assumed a ball should be lodged in between, and it can work as some kind of tilt mechanism.

Are you talking about the tilt switch rail to the right of the plumb bob?

It should have a ball in it and, if the machine is picked up at the front by a player, the machine will tilt.

This is an earlier, but similar, version.

ScreenHunter_316 May. 15 13.17 (resized).jpg

There is usually a 15/16" ball in there. It can be squeezed in without removing the rail. It makes the game tilt if somebody were to lift the game up to prevent the ball from draining. Not really necessary in a home use setting.

I remember Aladdin's Castle had a similar mechanism and the plumb bob. I too am curious as to why both were used together since they both would trigger a tilt if the machine is lifted from front.

Maybe, just maybe, the plumb bob hanging to one side only counts as 1 warning (even when it persists), while you want the game to end in that situation. Just as slam tilt also does not count as 1 warning. You slam, you end.

The roll ball tilt is for when kids put ashtrays under the front legs to make the playfield flat and easy to guide the ball with a magnet to win free games.