Pic1, that's an "old work" box that's held in place by the little tabs on the top and bottom pressing against the sheetrock. What's weird is that these are typically used when there's no access to the interior of the wall and NM is being fished in. In this case, there's access, so why use an old-work box? Maybe it was originally placed long before the current work being done.
(I use the term "work" rather loosely here.)
Pic3, it looks like there is indeed a clamp on the metal cable. It's a "Tomic" or "butterfly" type, but it's mounted inside out (screws on the inside), and it's probably for NM cable only.
It also looks like there's another cable sneaking down the wall behind the box. There's just too many wires in that box for only two cables. There's an unterminated black wire in the middle, so I guess the intent is to install switches or receptacles, too. No worries, plenty of room!
P.S. I just noticed -- those are 3/4" knockouts, but the NM clamp looks like 1/2".
[This message has been edited by John Crighton (edited 06-09-2005).]
