The Mauser threads started life in the late 19th century. By the time the 1891 Mauser rifle came out, the standard barrel thread was 12 threads per inch. The British Standard Whitworth Thread form is simply a 55 degree thread instead of the 60 degrees in common use today. The roots and peaks of the threads are rounded instead of flat. The metric system was not standardized until much later. ISO Metric thread form was not standardized, for instance, until 1975. The Germans, for reasons of compatibility, used 12 threads per inch for all their production. There is not a metric screw in a Mauser rifle. The action screws are 1/4x22. At the time they were first used, there was no NC or NF thread form either. If there had been, the screws would have been made 1/4x20 or 1/4x28.
Clemson