In the 1800s when Britain and the USA stopped keeping their units in alignment, they both defined conversion factors in terms of inches per metre (the inverse of the modern definition). However the conversion factors were slightly different. In the 1920s and 1930s, Carl Edvard Johansson used 25.4mm as a compromise between the two definitions when making his guage blocks, and this became a de facto standard that was later blessed as the international standard inch.

https://en.wikipedia.org/wiki/Carl_Edvard_Johansson#Johansso...