The 42311-390-000 spacer is outside of the actual wheel assembly. It is pressed between the left wheel bearing and the swingarm/chain adjuster, going through the sprocket carrier and seal. No idea why it only fits the '75-77 550F Supersport models, except their swingarms are also unique to this model.
I would get my hands on one from a CB500/CB550 K model (42311-323-040) to try - same ID and OD, and likely very close if not the same in length.
Just came across this that may offer more insight:
http://forums.sohc4.net/index.php/topic,148877.0.html