I own 3 '77 cb400f's, have an affinity for the cb400f in the '75-'77 model year range (not the late-80s cb400f though) -- the 1st bike I ever owned was a 1976 cb400f.
All 3 of my 77s have a tank looks exactly like that, but your guy probably painted the top because the top of those sun-fades. All 3 of mine look fine on the sides of the tank but faded on top.
So I asked the seller when I got my 1st '77 about the gas cap and he said "Honda got sued because several riders got burned due to the cap opening in a crash on the early flip-up tank caps, so Honda changed the gas cap." I didn't verify the 'lawsuit issue' but it sounds plausible.