Club 80-90 Forums

Hi all. Looking for the part number for my camper. It has the original gear box and has only a plug in it where the reversing sensor should be. I'm have looked a quite a few webs site but they don't go back to 1980. I don't want to take the plug out as I will loose the gear box oil in the process to get the size of the thread.
Any help would be appreciated thanks.

Brickwerks do the early switch, in the electrical switch section.