musket the dog
Registered
It would not have surprised me, if they had done it the other way round...
Metric threads, but imperial heads, to save the cost of changing tools!
I have worked on an Pre-war MG engine with exactly that set up, but with Whitworth heads. The French company Hotchkiss moved their engine factory to Coventry during the first world war and when it was taken over by Morris some years afterwards it was much easier to have hybrid bolts made up than it was to change all the machine tooling and to re-equip all the technicians (and mechanics up and down the country) with Metric tools. I learnt that interesting bit of history when I managed to shear one and had to make a matching replacement up from scratch