Considering the machine was built in the US in 1965, it's not surprising that the machine used imperial units. Based on Wikipedia's table [1], it looks like metrication had not yet begun in a large number of countries by 1965.

Also, if your goal is to build a device that calibrates other machines, and most of the machines in your country (and, likely, most of the machines belonging to the majority of your trading partners) use one system of measurement, then that's probably the system of measurement you build the machine to calibrate against.

[1] https://en.wikipedia.org/wiki/Metrication#Conversion_process