Thanks to Nick Smith for this copy of the manual for 1927 and earlier Sunbeam motorcycles. Well worth a read if you have one!
A very interesting picture here of the footchange arrangement used by the works Sunbeams in the Senior TT for their 3-speed gearbox. This was perhaps not Sunbeam’s greatest technical achievement and likely one of the reasons that after their victory in 1929 the Rudge factory was so dominant in 1930. The crash box from the hand change models is retained with the exception of a shorter main shaft that moved the clutch inwards and necessitated a dished engine sprocket. As can be seen, the lack of a positive stop mechanism meant that the lever is simply moved up and down to different positions in order to select a gear. A V-shaped spring does the job of the hand-change gate by engaging in a notched plate to hold it in particular gear with a spring-loaded catch that makes it harder to shift from middle gear. This is to avoid overshooting 2nd when changing up or down.
My own M90 has the same setup and it works well enough but Sunbeam soon moved on to their own 4-speed constant mesh type gearbox and later adopting 3rd party units supplied by Burman.