build/broadcast sheet help needed

On my 1969 Hamtramck plant build sheet, Line 3 (build codes section), under STR. WHEEL HORN RING, I have a 15 and a TT. I cannot locate what these refer to. Any help would be appreciated.

15 is the steering wheel style "sport"
TT is the color "wood grain"

the steering wheel part # is 4016 DTT