Brooks B72 saddles have four rails. The originals come with these crummy little plastic spacers that fit between lower and upper rails, that can tilt sideways and fall out, leaving the seat no longer connected to the seatpost. I figured such an epic seat needed a more epic rail spacer so I milled one out of 7075 aluminum. 5mm ball end mill for the rail channels (Brooks uses a nonstandard rail to rail spacing and a nonstandard rail diameter) and 1/8" endmill for cutting the isogrid. I roughed it out with 0.2mm of finish allowance, then did a finish cut, but you can still see toolmarks, so I have to ramp the mill into/out of the finish cut rather than plunging it. I flycut all surfaces for appearance.