The counterbored holes are for #8-32 screws that go to the motor gear box. I finally used my counterbore drill bit set. It's incrediby fast to use. For a one off like this it is easier than loading a end mill in to the CNC mill and cutting a circular pocket.

The adapter is machined from 3x3x1/2" 6061-T6. I chopped sawed it to size and then drill the holes and milled the circular frame on the CNC mill. Deburring was done on the new disc sander.