That's not too surprising. Each year after introduction they pinched the basic offering back. By '69, the fastback's fold down seat had become optional! That's just crazy. I'm sure it didn't help sales or customer Atteeetude to have that as an addon to the price.
Funny thing is, Chrysler still uses that pricing model. On my ‘06 SRT, lots of stuff was standard that first year that later became optional. Hood blankets, strut covers, base radio, etc.
They have alway used the model to get good sales in the first year on loaded vehicles, get them out for everyone to see on the street and then drop the base price in following years to keep the buyers coming in. They did the same thing on my wife’s ‘05 Grand Caravan, the first year of the redesign with Stow-N-Go seats.