Here ya go. From a cycle world article:
At one point, we were shown some of the equipment used to machine the piston bores of “Monobloc” calipers. Monobloc means what it says: made from one piece rather than bolted together or with piston bores machined from one side and then plugged. Just as you would think, there is a gear-driven, right-angle tool head that fits into the pad space so that it can then machine the piston bores from the inside, resulting in the lightest, stiffest caliper possible (no heavy steel bolts, no bolt stress, no weakness from multi-piece construction). As you would also expect, several passes and changes of tooling are required to finish the bores and seal-retention grooves.