Does you machine look like this?
If yes, I don't know the official Mills name for this model, but I've heard it called the Club Special, Gold Bars, Silver bars, or Ingot Front.
The serial number should be in the 55x,xxx - 56x,xxx range.
The stories I've heard; it was a special order machine or just a limited production for casinos and wasn't a real popular design, as a result, not a lot of them survived.