For the large top screw I have official meassurements - it's 18,8 mm x 20 t.p.i.
At the time, might the various designs have simply used standard British Imperial tool measurements, rather than some non-standard metric dimension (assuming say 18,8mm is not a standard)?
The practice is not confined to Germany, the PRF thread in the British "Red Queen" projectile (1950s) is metric diameter, Whitworth form and threads per inch.