I have just recently understood this myself. The seat frames are the same as far as I know. The difference is in the upholstery. The 69 and 70 have some wires or cords that are hog ringed down to the frame that pulls in the bolsters (or what are they called?) on the side of the seat backs and bottoms. The 69 and 70 stock upholstery has seams sewn in to hold these wires so you may hog ring them in place. The 71 and 72 upholstery does not have these seams for the wires. Hence they seem to be a little more flat across the bottom and backs. I just redid bucket seats for my son's 70 Monte Carlo with 72 upholstery ( some one changed out the entire insides of his car to a 72) and 1 of the seats I had was a 70 and one was a 72. The 70 had these wires installed and the 72 did not. They both came out fine with the 72 upholstery though and there is no other difference between the seats. The foam for 69 and 70 has a slice cut out for these wires and seams and the 71 and 72 do not. Just my experience.