SPID's were available in the '66 Chevy trucks. I have a few with them, as well as, the factory build sheet between the seats springs and the burlap, usually behind the driver side, but I have a few with the build sheet behind the passenger side seat back.
Here is a link to a fellow on the site that does them.
http://outintheshop.com/2.html
Quite impressive and for the money a great touch to a stocker, daily or show stopper.