[HO455]

Time to backtrack and disregard my drawing from the previous post. Now that I have the 2 seats disassembled and can compare them more accurately, things are not as they seemed at first.
It turns out that the seat back frames are the same height from top bar to bottom bar. The difference is in the curved mounting part on the bottom.
First photos the 71 bare frame with the 69-70 frame next to it. Not much difference is evident although the seat cover is hiding the difference in the bottom. And how they bolt to the seat base.
The next photo shows the 71 back attached to the 69-70 seat bottom. Notice how vertical the back is. The second big difference with this combination is the seat back will not go back nearly as far as the 69-70 did. If fact it is so far forward I doubt I could safely drive the truck.
The 3rd photo shows the 71 back on the earlier bottom. Notice how reclined the seat back is.
I tried to demonstrate the differences in the seat back angles but setting up an adjustable square as shown in the 4th photo. Look again at the first photo as see how the square compares to the seat back angle.
One more difference is in the gap between the bottom of the back and the top of the bottom. There is about 1 to 2 inches of clearance. I think that if the 71 seat back frame had its cushion and cover I would not have been able to bolt the 2 parts together without a lot of effort.