500/4 doesnt have a cluch switch and is differently wired for lighting to a 550 k3 from 1978. The 500/4 wasn't about in 1977 /78 so part of the detail is wrong. I did fit a 400/4 loom to a 550k3 honda, was pretty close match but with a few extra extensions for a few bits of the loom so a 400/4 and 550k3 combo of switchgear + loom might well work.