custom deluxe was one of the lower lines, just above the custom. The cheyenne and silverado trim packages were higher. Check out http://www.chuckschevytruckpages.com/ a lot of c-10 info there.
I was trying to figure that out as well, I have a 74 custome deluxe with A/C, chrome trim on the outside, etc. My seat is the first I have seen of it's kind I'm thinking it was ordered as an option or something?