I thought that those old Hofner necks were truss rod free. Could you try a nut off an old Gibson truss rod? Would they would be Imperial rather than metric?
Did some digging-around and found this...
They don't give an exact size but (in a different chatroom discussion) it's acknowledged to be metric. One contributor says he determined the size was 4mm, although the wrench had the number '5' stamped on the side?!
At least it may be a start.... best of luck!
My early 60s Hofner jazz box requiers a 6 mm wrench (see picture), and my 66 or 67 Hofner Club needs a 7 mm wrench. I cannot tell about the threads, but everything German use to be metric.