Many of the watch brands at the time sourced their cases/dial/hands from independent specialists.
As an example, a dial manufacturer may have a number of standard styles that were in a catalog. Zenith (or Omega or Rolex et al) representatives would select a dial, maybe adjust the style to their taste and order a run of qty X for their production runs for a particular model.
Later "in-house" started to become a thing as the major players swept up the little manufacturing companies under their umbrellas.