Dave Therhino,
Did some deeper digging and it looks like the tank
was redesigned to accept a Kevlar liner/bladder. From Flightglobal August 2003:
Although the -500 shares much commonality with the larger A340-600, it is unique in having a 19,930 litre (5,260USgal) rear centre tank (RCT). ... The Kevlar-lined tanks willbe certificated as a major modification by the JAA on behalf of the FAA.Aug. 11, 2003
Very prescient of the FAA as evidenced by the severe lower aft fuselage damage on an Emirates A340-500.
Emirates 407 A340-500 Tail Strike