Mine was the same mate, I bought the old girl on a 1998 R plate but when I wnt for parts she shows as a 2004 plate. How weird P38's were not built in 2004????? thought I bought a lemon. First of all do a full HPI check this can be useful. then contact DVLA there is a form you can download it's a V888 send it off with payment and they will send you all the history of previous owners and any plate changes.
I did this with mine and had shocking news but good news. My 1998 2.5 DSE was a Land Rover works vehicle at Solihull, The old girl had not even seen a main road until 2004 when she was first registered. It's worth doing the check my friend. Hope I have been of assistance.