Found some interesting speculation on Chopper Charles's Honda CX500 and GL500 Forum.
Seems that there was a Japanese company called Marusho
that built a line of V-twins in the late '50s and early '60s
that have a very striking resemblance to the CX500. The styling is different, to be sure. But they've got a transverse V, shaft drive, pushrods, and even have the cylinder heads twisted a bit so the intake ports point inward and the exhaust points outward.
Speaking of the CX500, I had a bit of a problem with mine. Turns out when I tried to fix the stuck needle, I put the float in upside down. The result was a bike that either wouldn't run or only ran on one cylinder. I got that fixed today.