I also think it is a sequence number of some kind. Karmann built the e9s and they stamped a number hidden by the Karmann plate affixed to the driver side A pillar. The 2800CS was numbered up through 9,399 and they started over with the 3.0CS for the next 21,166. There was also a piece of paper glued to the passenger side rear seat bulkhead to alert assembly workers to the vehicle configuration, CS/CSA/CSi/Euro/US etc. and these correspond to the four digit model numbers used just like in the 1600/2002.