The hub changed during the tail end of the K1 engines, which appeared up until the early K2 bikes.
The later hubs had a few more oil holes in them, in different locations, from the earlier ones. This was done to improve the clutch smoothness, reducing the 'grabby' in-out box feeling of the earlier clutches. I've got a post in the "Thoughts" section about it, recently updated (I think Steve put it near the end of those pages).
From all appearances, it seemed that Honda seriously overbuilt the earlier engines. There were things like hardened and lightened cam sprockets, hand-ported heads, hand-balanced piston sets, sized-and-shimmed transmission spacers, and a number of other things that slowly disappeared as the horsepower dropped in the later style engines. This is the main reason why so many K0 part numbers are different from the later bikes, while the parts usually fit just fine between the models. Adding a steel band around the clutch hub, to contain the torque and strengthen a hub that the engineers were unsure of under American abuse, would be typical of the changes that came and went. Drag racers used a similar trick: they would cut into the hub a bit to add the collar, then also run a tiny ball mill down the sharp edges of tee inside corners of the splines to stress-relieve them (it worked!).
