El Caminos were part of the Chevelle line and thus would be intermixed with all the other models during production, e.g. sta wgn, 4 dr sedans, convertibles, and so on. Often they would build certain models in groups but not always.
Numbers you are finding written on the car were done by line workers for easy recognition. They would write that number on some part etc and it made it easier for them to build the car. I had a 1999 Suburban K2500 from Janesville that had similar numbers on it. The control arms, rear axle etc had 256 in crayon and it was also on the firewall.
Here's links to interesting info..
http://www.bowlinggreenassemblyplant...-manifest.html
http://www.bowlinggreenassemblyplant...roduction.html