The screw thread sizes are in gauge but the length is metric, another fudged to fit metric system.
Here is where they came from
Here’s another
1" to a foot, god knows if they tried to make that metric?
Believe it or not there was some metric inventions, well one that I can think of.
There are some items that were invented in metric and have been in common use in USA equipment and worldwide for almost 100 years…
Here’s the part number for one 6206, still remember that from teenager years…
Some of the ginger beers here might know that one as well fixing cars trucks etc.