Curiously, Thunderbird model C K7s, which introduced FSB266 and copper interconnects were the first AMD CPUs to carry the 5-letter code, earlier aluminum-based Thunderbirds were only marked with a 4-letter stepping code. Since copper interconnects are much trickier than alu-ics to manufacture and the configuration of the copper layer is crucial to the chip's electrical properties it might be a reason to add a 5th letter into the originally 4-letter stepping code.
My guess is, the 3rd and 4th letters mark ASIC revisions, one for the revision of the silicon and the other for the metal interconnect.
Similar to, for example, the spin/silicon/metal-ic codes laser-etched on AMD/ATi GPUs ("A13" for retail R600, eg.)
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