Home was built in 1949, in suburban DC, moved in recently.
In trying to mount a few floating shelves, I ran into a peculiar (to me) situation. My stud finder couldn't locate a stud in my wall, so my friend recommended using "wall dog" screws. In putting the first screw in, I realized the drywall is quite thick, probably 3/4 of an inch. Past the drywall, there is a small gap, and then something metallic. it seems like this metallic area spans the whole wall. The other side of the wall is the exterior of the house, which has a brick facade. The drill bit I used showed no indication of insulation.
Any ideas what's up here? Many thanks!