Thanks Angus, As far as I can see from the parts diagram and by peering in through the selector opening there are no ‘dogs’ or other mechanical locking device to hold the rotor selector drive gear in position once it is engaged. The rotor selector drive gear and the idler are straight cut and the rotor selector drive gear slides on the shaft to mesh with the idler.
I would have expected to find some form of mechanical locking but I just cannot figure it out. Any information would be greatly appreciated.