With the introduction of the improved A+ engines from Rover, one of the the changes was a revised distributor and its mounting system and drive gear. The drive gear has a wider slot for the larger engagement dog on the bottom of the distributor. It will fit only A+ distributors. The drive gear must match the distributor being fitted to the engine. We also usually have a supply of these available in good used condition.
See 12G4499-USED for pre-A+ drive gear.