These joints are complete with the knuckle, nylon cup (21A423) and rubber boot (21A425).
If your suspension has dropped suddenly, or you have loud squeaking noise during suspension movement, then these knuckles are one of the first things you should check. If left unchecked, the knuckle can wear through the nylon cup, into the suspension arm, and ruin the casting that supports the knuckle joint.
Much cheaper than changing arms. Fits front and rear except later cars 1990 onwards that had a built in spacer for extra ride height clearance (GSV1264).
In 1984 when the Mini was put onto 12" wheels as standard several small details were changed. One was to include a thick washer under the knuckle joint (21A1845) to raise the suspension a little in order to gain wheel clearance and suspension travel. Others included packing behind the teardrop mounts, also to gain wheel clearance.
In 1990 when the cone spring specification was slightly changed (again) they decided to introduce a front knuckle joint that had this spacer included in it already. This was both to make assembly faster and also to prevent mechanics losing the spacer when repairing the front suspension and completely messing up Mini ride heights when they failed to notice that one side was lower than the other.