I'm lucky to have that museum just around the corner from me. They're having a raffle to give away a fully restored first year sandcast CB750. It's absolutely gorgeous.
My guess is the seller either doesn't know what they have, or they do. Do the research. But be careful: a LOT of people are rehabbing stuff to look like old original stuff. They've been doing it to Fender Strats for years.
Good luck, and let us know how it turns out.
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.