I strongly suspect that the only place that you are going to find it is at Clausing. Note that its description is Woodruff Key, Special instead of the usual Woodruff Key #n. And note also that it is the only part with a ** beside the bubble number that also has a Clausing Part Number. I have always assumed that the ** is there in error.
If Clausing has no stock, be sure to ask whether you can have a PDF or TIF file scan of the original drawing. And send me a copy to put into Downloads.