Getting back to wirraway's question, from Sir Chook's diagram it does look as if the double sprocket 9A was just an idler, introduced as a design change to gear down the drive so that the mower's forward speed was reduced. It looks as if ATCO spaced the tiny sprocket on the engine/clutch drive shaft further out, to accomodate that idler. It also appears that wirraway had put the idler on its pivot shaft backwards (larger sprocket on the inside, when it should have been on the outside), which would have defeated its purpose altogether.
