Most of the numbers from the Register of National Roads (1998) that did not appear in the 1963 list were for statistical use only, from what I can remember. For example, the ΕΟ110 in the 1998 list refers to what is now the Α6 Attiki Odos.

I have to refer you to https://community.openstreetmap.org/t/road-classification-system-talk-poll/74967 to see how complicated it is and how it led to a lot of disagreements between editors. If you have questions, you can start a new discussion in the community forum.