Ah, so probably around the x550 series.
The first number of the model denotes the price range of the unit.
e.g. out of the current units, DEH-
1850 is the base model unit (~$120)
The DEH-P
5850MP is a mid range unit, priced around $550
The DEH-P
9850BT unit is the top of the range head unit (~$1000)
This is a DEH-3550MP