The Far Side

Yeah, but that's just a guess. Looking further, it sounds like it was indeed added by the printer, not on the artist's end, but that the art they sent was just black-and-white, with instructions to the printer as to what to what white areas to shade:

https://thenib.com/color-archive/

Cartoonists don’t create these dots—they’re added during print production. Cartoonists started with pen and ink on paper. Starting in 1894, their drawings were shot as photographic negatives, which were then exposed under intense light onto photosensitized zinc metal plates.

To add tints, cartoonists (or often an assistant or colorist) roughly marked up their drawings, indicating which grays or colors should appear in which areas. Comics syndicates created a limited set of colors to choose from based on mixing tints at different percentages, and those numbers would be marked on the comic. Some cartoonists used color pencil or watercolors on their original or a copy as an additional guide.

In production, engravers took these zinc plates—a single black plate for weekdays, and four separate ones for Sunday color comics—and painted around each area that needed tone using a water-soluble material called gamboge. They applied an oily ink to a sheet in a frame somewhat resembling a silk screen called a Ben Day screen that was covered with tiny dots for the desired tint. The engraver then placed the screen over the areas on the zinc plate that needed tint applied and used a burnisher to rub down the Ben Day pattern. They then washed the gamboge off. They might have to do this dozens to hundreds of times for a Sunday strip.

It sounds like that analog process became a digital one sometime around the 1970s, but still had the "artist-annotated image" approach.