Replying to
@cstross The UK has also been on the metric inch since like the 1930s. Saved our bacon in WWII because American-machined parts *weren't* a few thou per inch out.
Of course, in practice both the US and the UK had been on the metric inch for a while before then because the (I think Swedish) chap making the gauge blocks got fed up with having to maintain two product lines and split the difference between the two inches.