Easiest way to tell them apart is the gear sensing position pickup located below and behind the gearshift rod on the LHS is present on the 21/28 cases. It is absent from the 18 cases and instead there is a single brass 'sender unit' which is for determining if in neutral. The brass sender looks in appearance similar to the water temperature thermocouple that sticks out of the cooling system thermostat.

how do i know this..... 2 surplus 18 cases and a hunt for 21 cases!

cheers,