I am pretty sure when I saw yours out of the machine the other day there were four indentations in the metal frame that the plastics are mounted on.  I may very well be wrong, it was only a very quick look, but as per the manual, switch 67 will be on or off according to where those indentations are.  If it is sitting in the indentation the switch will read open so the machine knows that the face is in the correct position (flat).  It sounds like switch 67 isn't registering the indentation properly or the switch is bad.

Is there anyway you can leave the playfield up, do the test and watch what it is doing underneath or get a mirror and watch whether the switch and falling into the indentation properly?

It definitely sounds like everything is related to sw67, as it mentions in the manual as well.  If that switch is slightly out of adjustment then the face will not work properly.

Remember these machines are just a wooden box full of 30 year old electronics.  The machine needs that switch to tell is what is happening with the face.  The switch reads as open or closed, not so much on and off and it all depends how that particular switch is wired up i.e. normally open or normally closed. It sounds like when the switch is closed, the machine will want to rotate the head until that switch opens when it drops into an indentation.

That knocking sound could be the switch hitting or rubbing on something due to be out of alignment????

If you don't have any luck with it, I have Saturday off and could have a look for you.