Sounds like you have a harness from a '79 KZ1000B3 Ltd. It has 7 wires to the ignition switch. It has the 6 wires you describe on the 6P connector, but there should also be a single brown wire that connects through a single connector. Since the '79 Ltd was likely built in 1978, that is probably why the seller assumed it was from a '78.
If you can only get 77/78 6P ignition switch, you will have to modify the harness wiring, to get them to work properly with each other.
You have to locate the single brown wire that went to the ignition switch. It should be near the 6p connector. Then Y-splice that brown wire to the yellow wire of the 6p connector. Before plugging the new 6p switch in, make sure all of the colors line up, and that the brwon wire on the switch lines up with the yellow wire of the harness (which is also spliced to the brown wire of the harness).