In the days of hand-setting you could buy books of dithering (half-tone) printed on plastic film which you would cut and press onto a page. Some brands were Letratone and Zip-A-Tone, but it was generically called shading film.

Here's a 1974 Zip-A-Tone catalog: https://www.peculiarmanicule.com/zipatone

Here's an example of modern use of zipatone for manga: https://www.youtube.com/watch?v=Q2U4EfKCfjI