See window stickers here:
http://www.chevellestuff.net/1969/misc/paperwork.htm
First one, ATL car, shows head restraints with a charge of 16.90. VIN makes it a November 68 car.
Second one, also ATL car, shows no charge for head restraints, but then head restraints are deleted for -16.90. However, do note that the base cost of the car is $17.00 higher. VIN makes it a January 69 car. Which would be contrary to the headrests being "non-deletable" starting 1/1/69. Maybe the order was input in December?
A copy of the TSB regarding the headrests is also on that page, just below the two window stickers. It says if a customer insists, the headrests should be removed and "the sales price of the option deleted from the total sales price." That language suggests that there was (at that time, TSB dated 10/10/68) a line item charge for the headrests "option" on every car.
That would support the format of the window sticker above - an automatic charge on every window sticker, then an offsetting credit if removed.
The $17.00 increase in the car base price on the second window sticker likely coincided with the 1/1/69 changes, and covered previous headrests "option" cost, thus eliminating the need to list it as an "option" for $16.90.