The C8AZ Accessory Headrest Kit was not available as of May 1, 1967, but does show as being available as of October, 1967. That is as close as I can nail it down.
It was not to unusual that an Accessory with the next Model year's Part Number was introduced before that Model year, particularly if it would fit the current, or even prior, Model year.
The Accessory Headrest Kit typically associated with 1967 Model (as well as 1966), carried a C6AZ Part Number. It was considerably different than the C8 Kits. Pictures attached.
By October of 1967, both C6 and C8 Kits were shown applicable to '67's.
Hope this helps.
Bob