I just went through it again and looked very hard and still couldn't find the infinite type writer.
Spoiler
So its in the shield keys rooms? And do you have to break stuff to find it? I thought I found it when I found this very suspicious brick block in the attic first snake boss room, However I couldn't break it. Lol at first I thought I was looking for the gun Infinite Typewriter then I realized its a infinite save thing I'm looking for. Thank You For Your Time Ps. I just realized you said once you get the shield key Implying that it could be in the tunnels