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If the wire is green there should be a pink one with it. This is for your backup lights. It is connected to the trans and a small rod attaches to the reverse shift arm. There may also be a single black wire attached to the 3/4 shift arm. This is for the emission system on a 72.
The one your arrow points to is for the single black wire. The is for the emission system. It tells the system you are in 4th gear and to allow vacuum advance. Very primative system and very few worked well most have been long disconnected. The pink and green wire are for your backup lights. They should go towards the back of the trans and into a switch that connects to the rear most shift arm (reverse arm). If your backup lights are wroking then they are connected. If not you have some work to do.