Can anyone identify this plug?

This wire looks violet to me. You did not indicate whether or not the CAR was a 1967 you just said it had a 1967 slant six that was replaced with a 1969 340. At any rate that IS the two bladed receptacle for the 1967 reverse light switch. The backup light switch screws into the transmission. The two wires should go to the same bulkhead connector which also contains the wiper motor wires. Of course this would only be true if the car was a 1967 and had 1967 wiring.