The boys are right. It is a Kick down switch. You said it was for the trailer. The switch was mounted there or near the carb, depending.

If you still have the 400, you DO NEED that switch. I have read all of your posts and never saw that you said what trans the 71 has.
The switch was not wired, you said, so maybe the trans was changed or someone was in the process of rewire. That switch is still available.

Keep up the good work.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn