apparently there is a engine code by the dipstick tube.

 

True
> Rovers North - Classic Land Rover Parts-Identify your Range Rover and Discovery

 

Take off the oil pan and look at the rear lobe on the crank. It will have 4.0 or 4.6 stamped on it.

This is the only for sure way to tell.

Also, not sure why you are hung up on the lack of a PS pump, previous owner probably removed it. You won't find a P38 or Disco without PS.