Had a 60's SAAB, built with a variety of hardware sources, various european companys
there are a variety of thread pitches , fine to coarse, though they are to be perhaps both 6, as you say, MM

5 is commonly 0.8 thread per mm, 6 , 1 per mm
though theres a number of 5mm holding on the faceplate of my Azonic stem ,
its more a coarse thread than standard .8, perhaps to hold better in aluminum, than a finer thread would.

Broad generalizations fail, measure the pitch.