Hi Mick
It would be perverse if the spindle encoder were wired differently from the other 2, as they all share the exact colours and are presumeably the same brand.
I can't think of how you could verify which are 0V and 5V (the only critical lines) unless you are prepared to open the case and look at the circuit board. If the encoder were mine, I'd just conneft as per the other two.
Maybe someone who uses a Denford could chip in, or perhaps Denford's tech support could help if you are nervous.
Allan