I can perhaps give some insight into this. One step in the mask making process was hand cutting the design into a masking film called Rubylith. Rubylith is made

I can perhaps give some insight into this. One step in the mask making process was hand cutting the design into a masking film called Rubylith. Rubylith is made of a backing sheet coated in a UV resistant red film. The design was hand cut into this film on a large scale then used as a “master” to expose smaller copies.