This is whats called an 'old work box'. The original box was removed or this was just used as new when the drywall was up already. The screws on opposite corners are fastened to wings in the back which extend across the drywall to hold the box, it is not nailed to a stud.

Box extenders should work just fine on this, it's just a regular box.

Thanks

If your walls were still open, this would be a good box to use.

Here is a link that might be useful: Click here