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According to the Kodak Pro Digital site "Connectivity" chart, a Silvestrr Plushot or Proplus cable release is required to use their DCS ProBack Plus back on Hasselbald 500c
Note: not all Kodak ProBacks can be adapted for mechanical MF cameras. The eariler model only worked on a 555ELD (direct mount) and Mamiya RZ (via an expensive adapter plate). The later version of the back is the "Plus" part.
I don't know the adaptability of other brand digital backs to 500c cameras.