Well, this is a little embarrassing.
After speaking with Cris, and measuring again with new, good quality calliper, the readings were still odd.
Umm; read the scale properly!
Feeling a little silly, it indicates male fitting, M68 x 1 - the Zeiss norm.
Turns out that Baader ClickLock one is out of stock anywhere I found it, with indications it would not be in stock until early 2022. However, APM produces a similar FastLock adapter in the correct size. It is on order now, and once received I can confirm here, in case anyone else looks at the same thing.
The prompt for seeking this was to more safely accommodate the Denkmeier Binotron.