I still remember that all 7s and IIRC all 8s works in Quake 3 Arena as a CD key too. I found this out button-mashing as a kid after my actual CD key wouldn’t take. Probably something screwy with their CD key verification algo that probably just passes stuff around internally as 1 or something. I’m sure this is some whole sub-genre of cryptography — finding inputs that break the verification calculations — but I don’t know the name of it beyond “cracking”, albeit “non-patching-based”.