I was just being stupid above. There were different plants across the country and they probably used up all the old stuff before switching at each plant, so it would be hard to know exactly when the switch occurred for your truck.
The front bed panel has the same hole pattern along with the piece at the bottom of the tailgate that ties the two sides together.
You could mix the two, but it is not an easy thing to deal with if you want a stock looking bed.
