As I'm currently waiting for my new steel floor pans to be finished off, I thought I'd have a trial fit of the gearbox cover and in doing so noticed for the first time the hole pictured. It's on the drivers side (UK). Can't see it being a cable like speedo or wiring otherwise it would just be a grommet. Foot operated dip switch, maybe? Seems a bit high for that. You'll notice there are screw holes either side of the cheese triangle shaped hole so whatever was there was screwed on. Pics are taken from the engine bay side of the cover.
I love having the foot operated switch, great for night driving...but sometimes you do (well I do) get out of sink with this and the clutch peddle resulting in a sharp gear change