[Help Request] Can't figure out what the hell is behind a wall where I want to hang heavy duty shelves.

Location: Brooklyn, NYC top story of a 4 story building. Wall in question is facing west and doesn't touch an adjacent building at all.

Original Construction: 1930's until recently renovated though

So I am trying to hang some heavy homemade wood shelves to be my entertainment center in my living room and am running into a snafu trying to find the studs.

https://i.imgur.com/k8VLlXR.jpg

This is the setup. The part of the wall that protrudes out is ~63" wide and sticking out ~3" from the wall behind. The shelves are solid wood, 60" wide, and weigh ~25 lbs. There's a lip at the bottom of the I was hoping to be able to use lag screws to anchor into 3 studs on the wall (assuming 16" O.C.) then tie an angle in at the top to prevent tipping. When I tried to find the studs behind the wall it was super weird.

https://i.imgur.com/97FaEeL.jpg

This is what I can tell is behind the wall from a combination of a strong neodymium magnet and a decent stud finder (Zircon HD55). The points on the wall are where the magnet stuck to the wall. At the bottom, about 12" off the ground I can see a ~2" horizontal member that spans the entire length. The rest of the wall feels like a combination of plaster/drywall, cement, and wood. When I use the edge/stud finder it's of little use. In deep scan mode it gets hits in a spot then doesn't when I a pass over the same spot. The only semi-reliable edges I can get are what I have labeled as feels like cement and the one horizontal member at the bottom.

I think there is a possibly two (2) columns of CMU blocks then some vertical piece of wood is tapped into it, one (1) vertical stud is in between them, then either lathe and plaster or drywall is tacked on to form a wall. When I knock on the wall its definitely 100% solid in the spots I have highlighted. Above and below those spots it also feels like it could be solid - possibly CMU blocks that the poured concrete wasn't vibrated and has air pockets? Or maybe the CMU blocks are offset from each other vertically? I'm really at a loss here, I've never seen a wall this confusing. I am the top floor of a building built in the 1930's New York and the unit was renovated before I moved in.

Any guesses or suggestions would be great. I'm trying to figure out as much as I can before drilling holes into the wall.

submitted by /u/DrewFlan
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from DIY https://ift.tt/30Syfyq