In the TNG episode "Measure of a Man", we see Data's travel bag opened to show various items of relevance. On it, it appears that Data has some other titles, names, or honorifics. It reads as “Lt.Cmdr. ???/??? Data”, where between his (known) name and rank there seems to perhaps be text (possibly a series of acronyms?) but it's not clear. Are these symbols simply patterns and line art filling up space, or is there some other meaning here?
I just rewatched the relevant parts of the episode on Netflix HD. That image is already a crop from a full frame, so I don't know that you'll get much more detail there. I couldn't, anyway.
My first thought on seeing it was that it read "NFN / NMI" for "no first name" and "no middle initial", which agrees with Kevin's observation in his comment. Data is his only name. Personally, I would have considered it his first name, with no other names, but official recordkeeping databases don't always manage to deal with the full complexity of even human names, much less names for androids and non-human beings.