If I remember correctly, there's a similar "notch" in the DOHC frames as well. It's there so you can put a jack or lift under the engine, unbolt it, and then slide the whole assembly out to the right.
I thought it was a problem with mine, but once again Honda was thinking ahead.
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.