Thank you. Yes, that gets us much closer!
So the Attic numeric system used for the number of lines (where Δ=10, H=100, Γ=500, X=1000) is different from "Greek" system used for the book numbers (where Γ=3, Δ=4, H=8, X=600), presumably because the number of lines was a much larger number. But the system used for the book numbers could accommodate the high line numbers as well. It seems a bit odd to use two systems right next to each other.