OK, I checked, although not too deeply. A few formulas have the notation "Used for pool balls" next to them ((greenish blue, royal blue, navy blue, turquoise blue) while others (more) have "For export and pool ball marbles" (turquise blue, green, lavender).
Hellmers was pretty precise in all of his notations, so it may be significant that he seems to distinguish "pool balls" from "pool ball marbles." Were full-sized pool balls ever made from glass? Not something I know anything about . . .