Well, BMW San Francisco tells me that BMW North America tells them that the part I need is not available separately, but only as part of the "cassette," which I assume means the whole frame of the sunroof. So that's not happening.
Unless someone here parts out a pre-7/96 318ti with a California roof, or I find one in a junkyard, a makeshift repair is all I can do....
California sunroof repair followup 
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