Mystery 401 Switch hole

[Bobbyweaf]

I noticed a dent in my 401 facia and I felt no wood behind it, hence the dent. When I took the facia off, as it all needs re-veneering I found a switch hole where the dent was and in the metal backing holding facia switches and instruments, there was an open ended semi circular aperture in the metal backing plate.

This was an original aperture and NOT a subsequent alteration. The switch had been fitted on the lower part of the facia between the Rev counter and the ammeter.

Dashboard diagrams and pictures show no such switch or fitment. Any ideas? Remembering the back plate is as fitted by Bristols in 1952. My original 401 also had no switch in the same spot and being a Bristol owner for 47 years, I've never seen such a switch.

Can anyone solve the mystery?