3/4 -32 tap and 23/32 bottoming out bits are oddballs. About 50 each. I just happen to have both.
update: my machinist is almost done with 2 brass pieces, no stepdown, 21mm OD top to bottom. After that and Jon's approval on the first 2, I will have him do 2 pieces with the stepdown to 18mm. One of the problems with the stepdown is it leaves the walls at the base of the colette very thin. A solution may be to eliminate the chamfer at the base.