Are you really sure you searched for everything?
((EDIT: I do not mean to be mean there ^^, I just meant: Searching can mean a multi-stage process))
First search for "Siemens NTIO" brought this in the top 3 results.
Which then as a document name and unit descriptor has a full Siemens internal product series number (K21-06.50) and the identification of "NKOD"/"NKODH" and "NKID"/"NKIDH".
Searching for those letters and/or internal product numbers leads me toward a myriad of websites with names like "http://www.hobbielektronika.hu/", hosting PDFs about modules or connectors. This particular website seems to sell a load of stuff, but I can't read it. But if they have the PDFs, chances are they have something or will likely know (e-mail or call!) how or where to get stuff.
It does seem to be a Landis & Staefa proprietary internal type, but with 100's of websites particularly pointing at repair or hobby use, I'm betting in any corner of the world there's going to be a few. Since I don't know which languages you speak, the last half of that path is up to you.
As a first hint, these people seem reasonably professional and have a lot of stuff in similar series, they hosted a NKODH module PDF in the first search I did after the PDF, which also showed the NTIO module connected to it.
Same EDIT as above:
I also opened an RS-Online which came up in the search, they have a lot of visually similar Siemens stuff. I had a meeting so I posted before I walked all tabs. But I hate their website, so I didn't try to find anything there, but you could try to.