These sets have two gearbox adjusters, one on the gearbox side (off-side) and one behind the chaincase.  Some bikes left Meriden like this and were often justified by competition needs that would prevent the gearbox from being pulled forwards by the primary chain tension, under high power reversals.  In todays world, where belt drives are fitted instead of a primary chain, the  requirement exists for greater rigidity of the gearbox mounting.