I believe that unlike GM, Chrysler only kept track of percentages of cars with certain options, at that time. Seems like "The Standard Book of Chrysler" has some of those numbers, but in a non-consistent fashion, IIRC.
Considering how Chryslers were being promoted as affordable upscale vehicles back then, plus their "Free Automatic Transmission" promotions, IF a Chrysler dealer had a manual transmission car of ANY type on the lot back then, it was either: a special order, ordered in order to advertise a "From ______" price (lower than normal price), a mis-ordered car, or a Sales Bank car they bought way back of its actual cost. Even the 10% level is probably very optimistic, I suspect. But Plymouths would be a different situation, by comparison.
In those earlier times, option installation rates were not really worried about, just that you didn't normally see some things. Dealers kept vehicles that would sell easily and quickly, not something they would have to pay floorplan money on after the grace period ended. Or HAVE to sell at or barely above dealer cost just to get it sold. Just plain dollars and sense, especially for the smaller dealers.
Just some thoughts,
CBODY67