I was thinking that OD was a factory installed option, but I don't think that's correct now. I have the '63 shop manual, the 64 and 65 supplement, and the standalone 65 wiring diagram and none say a word about an OD. The only one I have not checked is the Factory Assembly Manual, but it's very hard to search as nothing is in any kind of order. I did find this thread that might be of some help:
http://67-72chevytrucks.com/vboard/s...d.php?t=313886
Also you might look at the last pic on this site:
http://www.classictruckshop.com/club...over/drive.htm
Hopefully someone smarter than me will chime in shortly. Sorry.