Often the 4X4 box is a "bracket box" with mounting onto the side of the wood 2X4 (most construction here is wood-framed with drywall). That leaves one side of the box firmly mounted, but sometimes the other side moves or hinges. The tabs were probably "steamboats" which can help firm the loose side to the wall. Usually though, the steamboats are used for "rework" boxes whose only support is the drywall. The top and bottom tabs of the rework box keep it from pushing into the wall, and a steamboat on the two sides keep the box from pulling out of the wall.
Now you're probably REALLY confused. Pictures anyone?
Dave
Dave
