i'm thinking what you have is possibly a saginaw trans. that switch would be the transmission spark control switch they used for a few years in the '70's. it was somehow wired into the dizzy and would retard the spark so exhaust temp got hotter to get the early catalytic converters to work. they were usually unplugged by car guys and lots of guys would just plug that hole if the sender started leaking.
https://www.hemmings.com/stories/art...aw-four-speeds
https://www.google.ca/search?q=sagin...eqESa5NbZLKeIM