It means it’s a file photo, meaning, a picture used to illustrate a news/editorial story, but not a current photo—it’s one that was pulled from the archives/photo library.
Just to expand on pulykamell’s post: it was not the custom in the past to give individual photographers credit for photos. Some papers may have done so at some times, but most of the time a photo would run without credit. Moreover, if one was used (even all the ones that were printed) would just be placed in a literal file with the subject’s name on it. So there may not be any record of the originator.
I don’t remember exactly when the practice changed, but it wasn’t all that long ago in historic terms.
I wouldn’t know, either, but it must have been quite some time ago. Looking at the archives of the local papers in the 50s, it looks like it was usual to give photographers credit–at least the staff photographers.
It may have started in the 50s. I don’t often see it in prewar photos. But wireservice photos - the Associated Press, UPI, INS, etc. - were generally not credited. They undoubtedly were placed into files. So were little headshots that went along with announcements, from weddings to funerals to society events but seem to have been provided by the people involved without a newspaper staffer doing the shoot.
Wire photos even now vary much from paper to paper. Some just credit the agency. Others do agency and photographer. As someone who shot for the wires back in the late 90s, I always appreciated the ones who would credit.