I found about 2-3 of them on EBay. Just did a search on "cb750 owner's manual" and some were pre-1979. I don't think there were a lot of differences across the years.
Besides, real men never read the instructions, do they? :-)
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.