To set the terms straight, there were three interiors available with the B351: standard, Decor Group, and the Mach 1 Sports Interior.
Standard interior would be all vinyl seats, flat door panels with separate arm rests, and a black textured center stack w/the gauges. Carpet is the plain type with heel pad on the driver's side.
Decor group would add molded door panels, camera case textured panels on the driver and passenger side dash, and either knitted vinyl seats that mimicked the Grande' upholstery pattern (black or white only), or the cloth and vinyl striped seats (all other colors). The center gauge stack would still be the textured black unit from the standard interior.
M1 Sports Interior gave you the knitted vinyl seats with the contrasting vinyl pattern vertical stripes, carpet with the running horse pads (as seen in Sherriff's pics). The center stack would also change to the woodgrain as seen in both your pics.
If your car had the black Decor Group interior, then the door tag code would be 3A or CA
If it had the M1 Sports Interior, then the code would be 5A.
The 429 Megasite has some good pics of the various interiors.
http://429mustangcougarinfo.50megs.com/new_page_36.htm
