I had one of those universal-replacement ignition switches in a parts box, but I was cruising the web looking at switches anyway. My wife chose this one because the ignition key was "cute!"
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Massey ignition switch.jpg
Note the double nuts for sheet metal mount and the ridged neck, which seems to be designed to keep it from rotating in a wooden dash panel. That was a problem with the Dorman switch in the bin - the springs inside were so stiff that the switch would turn no matter how hard you cranked down on the mounting nuts. The Massey switch also appears to have a D-shape at the threads to help keep it from rotating.
I'm hoping that being a tractor part, there's some measure of water resistance as well. Tractors have to operate in the rain, anyway. $7.59 + $1 shipping from Amazon.
I was looking for rotary headlight switches and found what at first looked ideal: Standard DS-380, for an '88-'92 Pontiac Le Mans. The switches are everywhere for $20. Round hole, file some slots for the retaining ears, push it in. But nobody seems to have the mating connector, which looks nothing like an normal GM connector. That turned out to be because the '88-'92 was actually a rebadged Daewoo. I found some other names it was sold under in different countries and batted zero each time. It has blade connectors, so theoretically I could hook it up by pushing female spade connectors onto it, but I found this instead:
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Farmall headlight switch.jpg
That's a Farmall Cub rotary headlight switch. That's the early style; the later ones have a different knob:
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Farmall late switch.jpg
The Farmall switches are available in 6v or 12v. Some are three position, some are four. "Three" means off-dim-high, which I can wire to off-park-on. "Four" apparently has something to do with the charging system on some models.
I like the early style a lot. Downside: $45-$55+shipping. I'm going to check the local Tractor Supply Co. before I order, though.