I would guess there were COPO numbers for special paint. GM Production Control was a big operation and seemed to have forms and numbers for everything. There was a magazine article showing a copy of a COPO form which I copied and enlarged trying to read it. I sent a copy of it to Jim M and an ex Chevy engineer a few years ago asking if they remember it. Jim replied it is a "GSD-578" form. The title is "Request for Central Office Special Production Authorization." I was told by a restoration guy who worked on the Otis Chandler cars that Otis did a lot to get documentation from Chevy. As the LA Times publisher, Otis Chandler had some good connections with high level GM managers and had requested their help in researching the COPO cars. The guy said Chevy gave a pile of paper copies to Otis.
here is the COPO form GSD-578
http://members.aol.com/yenkochevelle.../copoformx.jpg