Mount for Bonniksen speedometer

[cmfalco]

The 1928 catalog shows the availability of an accessory Bonniksen speedometer that is located on the petrol tank but it does not show it in enough detail to see how it is mounted given that the two caps and clamp for them are between it and the headstock. I can't find a photograph in 'Black Ariels', or anywhere else, showing how a speedometer would be mounted in that location so a photograph would be greatly appreciated. Thanks.