Three main types of hub design:

Series 2 and 2A: unequal sized bearings (inner bearing larger than outer), 9/16" wheel studs on all but the very last of the Series 2As.  The studs are screwed into the hubs and peined over to secure them.  They have a nasty habit of rusting onto the wheel nuts and then unscrewing themselves from the hub, chewing up the thread in the hub as they come out.  Hubs damaged in this way can be reclaimed using the pull-through 'repair' studs, part number 561886. Alternatively the complete hub can be replaced with the stronger series 3 type.

Series 3 to September 1980 - similar design to Series 2 and 2A, but with much stronger 16mm pull-through wheel studs and larger nuts which seldom give problems.  This type of hub is directly interchangeable with the Series 2/2A type. If changing one hub from Series 2A to series 3 type you will have odd sized wheel nuts: 15/16" on the Series 2A hubs, 27mm on the Series 3 hub.  To solve this problem use wheel nut 576103.  This fits the series 2/2A stud but uses the same size (27mm) wheelbrace as the Series 3 nuts.

Series 3 1980-85: redesigned with equal sized bearings and double-lip hub seal, together with a different design of stub axle.  Not interchangeable with the earlier type.

To avoid oil leaks, it is important that the part of the stub axle where the hub seal runs is not rusted or badly scored.  On pre-1980 stub axles this part is a removable ring and can be changed, for post-1980 hubs the entire stub axle will need to be changed if the sealing surface is damaged.