NYC handyman here. Is it a building like you see in StuyTown? If yes, in the 60s for internal walls they often used wire mesh lathe secured on timber studwalls then sprayed concrete on them and then the walls were smoothed with a layer of plaster. Stud finders are kinda useless on this type of construction. 3" walls are not uncommon in these types of apartments. The plastic expansion anchors work fine for lighter items or togglebolts are a better option for heavier items.
On walls between apartments they used cinder block and a thim layer of plaster over the top.
gt272727 t1_ixdq8jz wrote
Reply to How to mount something safely on a wall of unknown material? by zeurydice
NYC handyman here. Is it a building like you see in StuyTown? If yes, in the 60s for internal walls they often used wire mesh lathe secured on timber studwalls then sprayed concrete on them and then the walls were smoothed with a layer of plaster. Stud finders are kinda useless on this type of construction. 3" walls are not uncommon in these types of apartments. The plastic expansion anchors work fine for lighter items or togglebolts are a better option for heavier items.
On walls between apartments they used cinder block and a thim layer of plaster over the top.