This is how we roll

"One story claims that a well-known prestigious car manufacturer would not hire a machinist until he had successfully demonstrated his filing skill. He was given an iron bar and an iron plate. When he was finished the bar had to be filed square. The plate had to have a square hole filed through it. If the square bar did not fit perfectly into the square hole no matter how it was turned, the machinist did not get the job." --- the home machinist's handbook.

dfariswheel followed up with

A similar test for skills was given by English custom gun makers.

You had to file from irregular metal a perfect 1" square, a triangle, and other shapes, and file two pieces of metal to such a perfect flat surface that they would stick together if pressed against each other.

One story I read was by a British soldier being trained as an armorer during the early, bad days of WWII. 

The country was literally expecting the Germans to land at any moment, but he and other men were having to try to file these shapes over and over to get them perfect. 

All they were going to be was standard parts switching armorers to do fast repairs of damaged weapons, but the people running the department were determined to not allow pre-war standards to fall.