For white lettering on the print, my guess would be the photographer wrote on the negative with ink. I've seen prints like that, and as far as I could tell, they all seem to be large format negatives. It makes sense if you think of it as both a way to identify the negatives as well as the prints.

FWIW, the ones I have seen were school pictures of my mother (mid 1930's) and were captioned with school, class and year (no individual names).