So I've got some news to report on the switch mystery. The Hodaka racing parts kit catalog shows the switch that I was trying to source and mentioned earlier under part number 918750R. Interestingly, the (obviously hand drawn) graphic in the book shows a wiring pigtail with 3 terminals. Seemingly not available anywhere under this part number. However the standard Ace dimmer/horn switch pn 908750C is available - it's clearly the same housing, attachment, buttons, and wire entry. Graphics in the book show 4 wires in the attached harness. So I went ahead and got one of those and, with some minor re-wiring, it will suffice for the subject part on the Harry Taylor replica bike.

The switch on Harry's bike

The new switch pn 908750C
Also, the frame and swing arm have returned from powder coat. Bike should be up on its wheels very shortly. Bike incorporates all of Harry's mods like the trick weld-on rearsets, swing arm reinforcements, and much more. Some mods visible to the naked eye, others not so much.
Looking forward to seeing many of you at Hodaka Days in a couple of weeks. I'll be on the search for more parts and info!
Jack Broomall