I am not an historian,but I am a theatrical scholar (MFA) and am very familiar with the themes of the play and the original film. From a costuming standpoint, it's tough to suss out much about the characters, even with the period taken into account. I would say, of nearly any classic film, THIS would be one I would pick as having very little in the way of costuming to analyze. The story lends itself much more to discussion of race, legality, and rhetoric.

But, to answer your question: During this time, it was standard for people to dress up for any occasion. Families wore their Sunday best to see a movie. It showed that you had status. So, a poor man and a rich man would have dressed about the same for jury duty. And men's fashion hasn't changed much in 100 years, so a simple suit and tie would have been the unifier here. This means we can not tell a lot about the class or income of these men based on clothing alone. This is a group of mid-to-advanced aged white men doing something of a business and civic nature.

There are some identifiers: One gentleman wears a garishly striped sportcoat, a hat (indoors!), with no tie. He also smacks his gum loudly. Regardless of era, we can interpret instantly that he is a bit laissez-faire,perhaps a huckster salesman. Another man wears no coat and has his shirt sleeves rolled up to accentuate his powerful upper arms. And, of course, the one hold-out, our protagonist, wears a white suit coat. A classic indicator in any film of "the hero".

We could determine some variance by seeing who is wearing a tie and a coat and who is not, but we learn in the first few minutes that they are in a un-airconditioned room, so this only indicates who handles the heat better, or who is quicker to dismiss the formaility for a small bit of comfort. Because the film is black-and-white, we can tell even less about their individuality. If one character had on a neon-green tie, that would say something, but we'd never know it given the lack of color.

Not much of this says anything about the period, however. A jury today would have some people in less business-like attire, and there would be women and people of color on the jury. But, here, the clothing says very little that could inform us about the plot and the goals of the characters in this time, in this place.

I DO think it's great that you are noticing and inquiring about costuming. So many films take on a whole new meaning when you begin to ask yourself why any character is dressed the way they are. Keep that up!