And GM never "placed them" they were more of "discarded" there as part of the assembly process by the guys on the line, correct?
I understand some were attached to parts like seats so then ended up in the right car at the right time.
Ditto. Assembly plants didn't "hide" them in a car to be discovered years later in a treasure hunt. Varies with year and plant where one might find one, two, or even three. Sub-assembly stations would attach them to the door panel, on top of the gas tank, stuff them in the headliner, cram them under the driver side footwell, hog ring them to underside or back of seats, anywhere they could either by plant SOP or just so they didn't have trash at their station.
1969 was a transition year for many plants going from the BODY/CHASSIS BROADCAST COPY sheets to a single broadcast copy (commonly called a build sheet today). Fremont and Framingham were using the common build sheet as far back as 1965 that I know of along with things like a PASSENGER CAR BROADCAST COPY or PRODUCTION BROADCAST NOTE.
To date, I've found that both KC and Baltimore used both BODY BROADCAST COPY/CHASSIS BROADCAST COPY paperwork as well as build sheets. Some 1969 are found on my old website ChevelleStuff.net on page 1969 Chevelle Build Sheet
. On that page, there are links to other years (1964 thru 1972) showing examples found for various Chevelle plants.
I've had contact with gentlemen that have found Buick build sheets in the KC Chevelle. Another found 3 build sheets in his 70 - problem was they were for 3 different Chevelles (in sequential order) and none were his car; one a 13669 Malibu sedan, one a 13680 El Camino, and one 13337 Chevelle sport coupe.