Sven,
A "self help" book sold at a major DIY shed has a picture of what looks like a handybox (stamped onepiece thing with rounded corners) inside a wall
Is it possible that what you saw was a plastering with a 4" square box behind it, they come in single and two gang types for 4" squares anyway
The part you would see would have rounded corners very close to the dimensions of a handy box.
IMO a handy box is anything but handy, very limited in usefulness if you follow wire fill requirements.
Bob
