My house is so old, the boxes look crooked because they are plumb and square. The walls are crooked [Linked Image]

Seriously, though, the original boxes (black in color, and seemingly made from something that is bulletproof, actually just good old heavy steel) were mounted with a piece of wood from stud to stud, across the bottom and the top (not the thickness a full stud, though). They were screwed in with wood screws, one on top and one on bottom, diagonally.

They are very difficult to remove. Keep in mind, these are the "old work" type of boxes.