I don't know specifically but this prepress history gives a qualitative idea of how it was probably done: https://www.prepressure.com/prepress/history/events-1

I don't know specifically but this prepress history gives a qualitative idea of how it was probably done: https://www.prepressure.com/prepress/history/events-1980 https://www.prepressure.com/prepress/history/events-1980