Back-up light switches would be operated by the reverse lever, not the 3rd & 4th lever as this trans.
Switches were installed on the side cover around 1973-74 as part of the emissions control system to control ignition timing. On automatic transmission equipped cars there was a pressure switch installed in the high gear fluid circuit that would activate a solenoid which would allow vacuum to flow to the vacuum advance diaphragm. The intent was to deny vacuum timing advance until the vehicle was in high gear which was supposed to reduce oxides of nitrogen at low speeds.
There were a lot of bad tune-ups back then because guys would set initial timing and neglect to check for total advance. If you did not check timing with the switch closed, there would be no vacuum to the distributor and total timing would be low. It was common for the circuits to have switch or feed problems ( it had its own fuse) and the end result was lower performance and loss of fuel economy.
Tom
M-21 close ratio switch is for backup lights
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