Thank you for your detailed comment and references Marianne, they are certainly worth serious consideration. I have found several examples of the "M/F/O" variant in the Internet already (albeit in India, although the quality of the English looks quite good on the whole, such as: https://www.greensurgicals.com/career.php
), and several in PDF files from Canadian sites (such as from a site called http://cisvvancouver.ca/
), which I won't link to here.
Yes, you are correct, it was EOE M/F/D/V!
The sites I found as references were books.google links and the views I had were slightly small; when I enlarged them I realized that what I thought was an "O" was a "D". But even though I was mistaken, the fault lies partly with google, because if you do a search for the following text, the hits which attracted my attention on the very first page actually read "M/F/O/V", even though on the actual books.google page one finds a D (a google scanner error perhaps...?):
"m/f/o" job vacancy
As for "M/F/X" there certainly seems to be quite a lot of hits in google for that. The majority of them seem to be concentrated in sites originating in Belgium. Nevertheless, I think I will go with "M/F/X", because it seems to be in use in the job adverts of some major organizations, such as BNP PARIBAS.