I bought from vintagecb750.com
The on/off switch one is listed as the only one they currently have that will for sure work with the 75 cb750f, though they note the original 75 cb750f did not have an on/off switch.
I replaced the loom the same day. Everything on the loom side is new. If the on/off switch is useful then I'd rather just use that one without modifying the 77 one that came.
Sound like a plan? Might pick up one of those kits anyone, though.