Can you tell what fabric these work shirts are made of?

Love these mid-century two-pocket shirts on Neal Cassady and this other guy here. They look soft though—so like chambray, or chamois or linen or just very worn denim? I really can’t tell.

When I find shirts like this now they are stiff material and boxy fit like Dickies or Carhartt. Trying to find something more like in these pics.