According to airlinecodes.co.uk
, J6 was used for CAAC aircraft. AFAIK, CAAC was sort of the "Chinese Aeroflot" and its successors were entirely different companies, meaning that Boeing would slap entirely new codes on the planes going to those companies. That also applies to Air China which took over the 747-4J6M you found from CAAC - but a Boeing customer code never changes* once an airplane has been delivered, so the ex-CAAC jet remained a 747-4J6M.
* I know of those 707s way back in time...
[Edited 2005-02-15 01:01:38]
Don't cry because it's over, smile because it happened.