Agreed they are not seen ,,,, 1/4-20 is really common in the US. maybe one day the real thing will pop up and all these questions will be known... A m6 version wont be hard to do ill have a handful for our European friends soon.
If this part is not from a German firearm, but sourced in England in 1976 , using vintage scrap parts, it's likely to be 1/4unf , however if it is indeed from Europe, it will definitely be m6 .
Engineering shops didn't really go metric fully until,the mid eighties in the uk .
Having said all that , as the thread is invisible on the prop, does it matter ?