So Ford used a fine spline for the first five years, a fine spline with a spring cushioned disc for four more, then went with the coarse spline with solid disc for the rest of the run. Ford expunged the 2000-3000-4000 fine spline parts from the books, and from the world, insofar as possible.
What was happening with the 5000-6000 shafts and discs from '65 thru '68?
Great post! I got to look up the meaning of "fretting corrosion"! And, better yet, I got to daydream about treasure troves of fine spline parts that escaped the jaws of the scrapper.