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Try Ottawa Fastener and Supply in Bells Corners? Not sure if they'd carry something like that, but I've always had good luck finding oddball parts there.
Try your local gun smith.
I have tried fastener stores and they look at me like I have three heads.
The screw thread pitches are primarily for firearms.
It took a week for one company to find taps and then they said they were custom made.
Let us know how you make out.
257 Roberts
If you are interested, I have some 6-40 x 5/16" or 1-3/4" round head, slot drive but not countersink.
Mind you, I may have some countersink brass 6-40 x 1/4" (UNF only) in a countersink bolt. I know they are somewhere in the warehouse, but I would have to find them. Will brass do?