The significance of the "RP number" is that it indicates which date and where the car was built. The complete RP number is in effect a production batch number. For many spares, Citroën and other parts lists (e.g. GSF) will show something like RP6615 - RP8141.
The code on the car is in the format 9999 XX, but the last two characters, indicating the factory, are generally not quoted. The ORGA number (otherwise referred to as the RPO) is the first numeric part of the RP number (7015 in the example below), and is a code for the date of production starting from 8th November 1976.
Later cars may have a leading 0 in the ORGA to make a 5 digit number, this does not affect the date calculation. Still later cars (after ORGA 9999) will of course have a leading 1 which is used for the date calculation.
ORGA/RP Number Location
Xantia (up to 1998)
The RP number (and the paint code) on cars up to about 1999 is stencilled in yellow or white on the offside of the engine compartment bulkhead behind the suspension sphere. The last two characters (factory code) may be separate from the all-significant RPO number. In the photo (right), the RP number is 7015 U7 and ERYMR is the paint code (for "Vert Vega").
XM (up to 1998)
The ORGA is of the same format as for the Xantia, but is located on the LH inner wing, near or under the air filter.
Most Peugeot/Citroen vehicles built after 1998
On cars built from about 1999 onwards, the RP number is on a label in the drivers side 'A' pillar near the tyre pressure information label and is often preceded by two zeros e.g. 008293CJ, that are ignored for parts purposes.
If you can't find the RP number on the car (sometimes they have been painted over or stickers removed), the ORGA can be found from the VIN using the http://service.citroen.com site. If you enter the VIN and go to the vehicle characteristics page, the ORGA is near the top of the list (usually the second line?), but may have other digits on the end which you may ignore for this purpose.
The last two characters of the RP number indicate the factory where the vehicle was built:-
|CA||Aulnay sois Bois||France|
|CJ||Rennes la Janais||France|
|U7||Heuliez||France||some Breaks (estates), AX + Saxo-Electrique|
|U9||Sevel Nord, Valenciennes,||France|
To convert the ORGA (e.g. 7015) to a build date, add this number (of days) to the base date 08/11/1976. It's very easy to do this with any spreadsheet software (e.g. Excel) or the table function in a Word-processor (a little less easy):- Enter the ORGA in one cell (e.g. A1). Format 2 more cells (e.g. A2 and A3) as date in dd-mm-yyyy format. Enter 08/11/1976 in the second cell A2 and the formula =A1+A2 in the third (A3). Using the example above, this calculation should give a build date of 23/01/1996.