What is the switch on the bottom side of steering column on a 1979 Ford E250 Econline Van?

My van will not shift into reverse or park and will not start. I have replaced the neutral safety switch, checked the shift cable and shift cylinder on the steering column. While doing that is see a sliding electrical switch on the bottom of the steering column which works in conjunction with the shift lever. What does this switch do?

That's the neutral safety switch… It should have an alignment hole or mark for setting it in park, that lines up also when in neutral.