Maybe this helps You Please note letters E, U, L, A, O, S, I, Z and numbers 1, 0, 5 are never used in the Windows product key combination. Or a bit more clearly, here is a list of characters used and not used in Microsoft product keys: characters and numbers used: 2346789BCDFGHJKMPQRTVWXY characters and numbers not used: 015AEILNOSUZ
Minor add-on: character "N" was added with: Visual Studio 2015 - retail and my MAK keys use it from Windows 8/Windows 2012
AFAIK, all keys starting with 8.x use one letter "N". Note I said "one". There's one and exactly one letter "N" in each product key starting with 8.x. Win7 and prior have zero "N".