In the Haynes manual for these bikes, towards the back, there's a listing of frame/engine numbers denoting models and years and so forth. It's kinda handy, and will tell you if the engine has been swapped out. I noticed that my bike is in the right ranges (1979 CB750L).
Check your local library: they may have a copy there, or perhaps a cycle repair shop or megastore. Should run you between $20-$30.
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.