Upper C9ZB-63673A38 , that 7 might be a 1 its hard to read
As I suspected, the Ford base number - 673A38 - is a group of seat components, in this case it's called "Rear Seat Silencer", used in sub-assemblies, and nothing to do with seat coverings.
Lower C9ZB-6363120A
This Ford base number is 63120 which is listed as "Front, Center, Opera, & Rear Seat Cushion Springs", again, not seat covers. The -A signifies more than one (there's a -B).
The rest of the numbers, "C9ZB-63xxxxx" defines the decade "C" = 1960; the first year used "9" = 1969; the Ford vehicle "Z" = Mustang; the engineering group "B" = Body; the body style that used the item first "63" = Sportsroof.
This info was found the the 1975 edition of Ford Car Parts (aka MPC) in the text section, general information, page 8. Neither number is listed in the general text or illustration sections.
Jim