The international inch is defined to be exactly 25.4 mm. It is approximated to 25 mm as a "metric inch".
A metric inch was used in some Soviet computers when Soviet engineers built unlicensed copies based on blueprints drawn in the US. Clones existed of many machines like PDPs and early IBM PCs. The computers would look like machines made in the US, but some of the components, mainly those using longer slots or more pins, were not interchangeable due to the non-interoperable nature of the metric inch. Shorter-slotted or lower pin count original hardware was often forced in by hand if needed, as such measurement discrepancies were within tolerance; longer chips, boards, and cables, with differences that would mismatch contacts, required converters, often home-made. Because microelectronics were a rare and prohibitively expensive deficit in the Soviet Union, computers cobbled together or upgraded with mismatched original, custom-built, and cloned components, more than just common, were actually basically the norm.
The similarly derived metric foot (300 mm) was once used in the United Kingdom for some expensive materials. It was also briefly the unit used in the trade of timber when neither feet nor metres were used.