When starting as an apprentice at the ROF straight from school I was faced with these funny measurements - millimetres!? Was not taught anything in metric at school.
The reason for using metric at the ROF was that most drawings were based on Bofors designed guns.
Then there was the mystery of different thread sizes and standards. unc, unf, bsf, bsw, bsp, ba, buttress, acme, square and even a 'bicycle' thread among others!