For the holes for the pistons, it's a matter of drilling them out. Simple. You then need a slot cutter to cut the O-ring groove and I'm guessing the same tool is used to cut a passage behind the two pistons (just an undercut under the bridge between the pistons). I'm not a CNC/manufacturing/tooling expert, far from it, but cutting a single piece caliper, by reaching into a deeper hole, possibly requires a longer stem on said cutter, which could influence the cycle times - the longer stem will be more flexible, so tooling loads need to be lower to...