Avram,
lookin at pictures of R Kotler and others from 30s to 50s, Rav and other Rabonim are dressed differently from others. Others wear whatever was worn by reasonable people at their time – jackets, hats or caps, depending on time & place. 1936 photo of funeral of Rabbi Yeruchom Levovitz at Mir, most people are in lihjt-colored hats.
You may also distinguish between “official” and causal photos. I see a yeshiva poster of individual photos for Mir in 1920s and everyone is in some sort of a hat. In summer camp, nobody is in a hat.