I think there's a bolt that holds the right side of the axle in place. This is not the nut on the end: this is a bolt that squeezes the right side. Check
http://www.oldbikebarn.com and look at the microfiche for the rear wheel.
I admit my info may be incorrect: I've been without my CB750 for well over a year now. Shoot...I can't remember what I had for breakfast last week.
Let me know if this helps. If not, we'll dig into it a bit deeper. Good luck.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.