Zeiss Ikon would do convertions from what we know as "black-dial" to "colour-dial". Supplement II of the service manual referring to the "fully synchronized" cameras says:
The synchronizing device cannot be inserted at a later date outside our factory, since the entire shutter mechanism must be exchanged. Cameras without full synchronization, therefore, should be sent to the factory.
I've seen one or two on eBay that should be black-dial according to their serial numbers, but seem to have been converted. Any camera with a PC socket but with a black-dial speed knob and serial number will be a repair man's bodge, though.