Before DTP, paste up involve pasting content onto boards that were then photographed (with a stat camera) to create film negatives for printing plates.
We'd stepped into the future.
Maybe this is just the process used for designing the layout, which is then used to re-flow and re-print all the "fragments" so they can be remounted and photographed cleanly?
This video shows a bit of the process such as the Rubylith sheets used to create masks for each color: https://www.youtube.com/watch?v=fxddg3dNKgw