MarkG
Yes the DP stands for DishPro. A quick way to tell if a E* receiver is DishPro compatible is to check the model number. If it is 3 digits long (eg. 301, 721, 811 etc.) it's DishPro if it is 4 digits long (eg, 5000, 4700, 7200 etc.) it is a legacy receiver.
DishPro receivers will work with any Dish Network LNB/Switch, Legacy or DishPro.
Legacy receivers will work with Legacy LNB/Switches natively. They can be made to work with DishPro LNB/Switches with the use of DishPro/Legacy adapters ~$70 each, or by using a DPP44 Switch.