Hi,
i have a spindle for the rear brake pedal with left hand thread on the outside (so NOT in the frame)
Never seen that before, or is it more common ?

Yes, Rigid and plunger twins used them.

Yes, Not required on an M20 at least as the normal R.H. thread nut locks up against the central collar and washer and there is no force exerted on the nut that would undo it...Ian

email (option): ian@wright52.plus.com