Quoted from Theganch:

That is not switch #50.
Switch #49 & #50 are stacked one above the other underneath the lower left building.
They are labeled as Left Kickbig (top) and left Kickbig (bottom) respectively and sense when there are two balls stacked on the left side.
That “switch” you are seeing is labeled very poorly on the manual, but is merely a one-way wire form. The sharp angle of the wire facing the interior of the playfield is meant to stop a ball from entering the ball lock area that doesn’t do so from the upper building. It is a proper “rollover” angle on the opposite side of the wire as the balls are ejected easily. That blade underneath is just to keep tension on the rollover wire and have it return to it’s original position after the balls have rolled over it from the lower left building eject.
I hope that’s a clear enough explanation.