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if the difference between the diameters is so great that the bearing bottoms out on the flat face of the seat before the collet has shrunk enough to grip the steerer
For some reason I was initially thinking about it upside-down, as if this old fork had an integral tapered seat like a modern carbon one. Of course there's no need for a gap under the 'race' - if the proposed slot does any work, it'll be closing up as the bearing moves down on it. Now that I look again, it's clear you're saying you'd need a gap between the bottom of the bearing and the 'race' - I'm getting sick of the scare quotes, is there another name for it? Rather not confuse it with an actual crown race
Kimmo
Google "split crown race". Loads of cartridge bearing headsets come with split crown races, the chamfer transfers axial load into radial compression of the "race" (collet) so the higher the axial load, the tighter the collet grips the steerer. This is also how the preload adjuster at the top solves the issue of the upper bearing collet not being a press fit on the steerer.
The simple machine which does this is called a wedge (in this case wrapped into a cone)
Of course, you could be right, if the difference between the diameters is so great that the bearing bottoms out on the flat face of the seat before the collet has shrunk enough to grip the steerer. You'll be able to predict this by taking appropriate measurements (will probably require a feeler gauge to check the clearance between the bearing and the flat face of the seat) and using trigonometry.