I've run a search, but find no real response.

Is breaking them off the only solution?

I've turned and turned and seen no progross on shaft. I've got the replacements (do they screw on? press fit? Other?)

('69, w/o Air)

Breaking them off is pretty much the only way. Vice grips work well. You'll need new ones to replace them with. To install new ones, you push them on but I'd put a little epoxy in the hole first to make sure they don't come off when pulling the vent open. Heating them up with a hair dryer will make them easier to push on.

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von '69 300 Dlx SS TC #15 ACES #1575 HSCA #5
My '69 SS (http://mywebpage.netscape.com/jerryacheson/vons69.html)