It's early and I need caffeine, but I'll try to explain.
Right up to the advent of electronic prepress (which means the necessary preparations for printed matter that occur before printing) and past it in certain instances, color separations had to be done photographically. Sometimes these separations required masks if some areas were to be included in the printed matter and some were not, say for instance a bouquet of flowers on an otherwise paper-white background. Did that many times for those store circulars for Valentines Day or Mother's Day, talk about tedious try hand-cutting a mask involving Baby's Breath flowers, lol.
There were two kinds of masking film that I recall being still in use in the mid-late eighties, amberlith and rubylith. You'd take an exacto knife, which is a pointed razor blade mounted on a handle, and cut an outline around what you wanted to mask off, and the amber or ruby color not within the mask area would then be peeled off of the clear plastic substrate. These were sometimes stacked many layers high on top of the base page layout, kept aligned with registration marks.
At the time, there were Varityper computer typesetting machines, and type for circulars was set by computer but photographically developed, run off of big rolls of photosensitive paper. This was allowed to dry, then also cut with an exacto knife and run through a wax machine, that applied wax to one side, and the wax allowed the type to be "pasted up" into position on a given page layout. The process really was pretty elaborate, given the ease with which such things are now all done on a PC. That slowly came into being, beginning at about the same time. The advent of PostScript allowed the first efforts with reliable representation of typography, then color images.
I've been running my entire career, to stay ahead of something, either technological advances or offshoring, lol. Managed to do very well despite that, with the exception of this recession/depression, which caused me to have to shut my business down.
Hope this made sense, as I said I've yet to be sufficiently caffeinated.
That was an excellent explanation - I am in the trade as well.
Thank you for the detailed explanation.
My experience (very limited) with physical typography and page layout was limited to pasting up strips of Linotype-set copy with hot wax on a light table, with a special “point marked” ruler.