From the 8 sample guns I have measured (all 1917 or earlier) , I would descibe this thread as 5mm diameter x 0.9mm pitch. The actual major thread dia. of the screws is 4.70-4.75mm which is a little smaller than the modern ISO M5 x 0.8mm (normally around 4.8-4.9mm).
It's known that prior to the ISO metric standard of 1963, Germany used the French SI sizes in 6mm-12mm only . Outside of these sizes the Germans used their own threads.
My measurements don't equate to any of the British Standard or British Association threads.
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