When your XM left the factory it was stamped with a 4 digit number to identify the date on which it was manufactured. The number is called the RP or ORGA number. It is used when ordering new parts and when trying to access maintenance information to help identify the correct parts/information.

The number is painted on the front nearside (passenger or left hand side for right hand drive cars) door pillar just below the top hinge. Open the door and look for the door hinges.

This website translates your car's RP or ORGA number into the date of manufacture.

http://www.cats-citroen.net/citroen/organumber.html

cheers

noz