...on a related note, but not answering your question so sorry, where does the horn wire exit the handlebars?  It’s obvious on my later Indian but I see nothing on my 101 handlebar center casting.

Thanks John!  (....and for some reason I have no holes like that in my casting)

Later Indians has a taller cover for the button, you might have to trim down the repro plastic isolator in order to fit your cover.  Here's a picture of 2 buttons, one a original button apart and a faithful repro one, assembled. The hole in the handlebar base is about 10mm (0.39") and there is a rubber grommet for the wire (2 in the picture). The hole for the button isolator is about 7mm (9/32") The screws are 5/32"-?tpi and 1/4" long.

On the repro button wire, the distance from the top of the wire butt down to the plastic isolator ledge is 6,35mm (1/4") it's a little less on the original from wear I assume.

Some handlebars has buttons on both sides of the handlebar, Fours does not have a valve lifter as the twins has, so early ones without the key switch have to have a magneto kill button. Some sport and racers need (or customers wants) a magneto kill button.

Earlier bikes may have a horn button clamped to the handlebar and the wire routed on the outside. Some bikes might have been ordered without electrics.