Welcome to the wonderful world of Ford codes.
AXEL, the Norwegian auto mechanic, told me that Ford did that A LOT of that on AXLES, fighting to use a DUEL code system that actually was not a DUAL system at all. YOU'RE seeing a solution where Ford installed something in YOUR car that was not an option for that year. So they "improvised" for the codes (I got a similar situation for a 66 "K" code convertible using 65 interior trim and exterior paint codes). Call it an anomaly, call it odd, THERE is no simple answer to THEIR codes.
Why Ford did not use the special order code combined with the DSO is a good question - a question with an answer lost in time.
Jim