roughly 2-3". It's a fairly old building with old elevators, the whole thing is scheduled for a gut renovation starting in a few weeks. I wouldn't be surprised if there was some sort of part missing there, the gap is awkwardly big. If it is missing something, they're probably like "fuck it, the whole thing will be replaced soon enough anyways".
Wasn't a big notebook either, I like the smaller ones from Muji.