Here is a pic of the base of the 603, as you can see there is no center pin on the base to connect up with the center pin of the hotshoe on the body, when the trigger is seated in the hotshoe, the top pin is set about 1/4 off center, this is the issue with the 603's.
I believe for both models of the 603's the base pins are the exact same, the difference is the cable that comes with the set, the Nikon packaged one comes with the correct sync cable...
Quote: You can take a canon flash put it on a Nikon or Pentax
It's not a flash issue, it's the circuitry of the 603 trigger that breaks the connection, circuitry that is canon and nikon compatible but not pentax...