I worked in a machine shop many years ago.This shop produced several Space Shuttle components that were mostly full-floating 3-way air or hydrophilic piston type valves iirc.

These valves valves required extreme tolerance control which, in that shop and at that time, could only be obtained by the use of an "optical comparator" device and a NASA approved and supplied "standard" with which to compare them against.

My only involvement with the production process was "culling the good from the bad" by the careful use of this comparator device. I was, at ~ age 22, one of only ~ 2 other employees who had eyesight good enough to use the device with an acceptable level of repetitive accuracy. You couldn't use this device for more than about an hour before you eyes "gave out" on you.

These valves were tested by at least two of us before they were sent out (and more than a few out of every batch were returned for being "out of specs"). I vaguely remember that only ~ 1 out of 10 or so of these valves passed the shop's "final" two-stage inspection.