Razer Blades?
Right above the slit was a tiny little label that read "razor blades." Obviously, the homeowner was confused as to what that even meant since that wasn't something one would see in the 21st Century. So, they decided to follow where the hole led to...
A Bunch of Razor Blades!
Well, that makes sense! The homeowner found a bunch of rusty razor blades in their wall. Apparently, this was pretty common back in the day. Razor blades were deemed too unsafe to throw in the garbage, so instead, they opted to throw the razor blades into the walls of their house.