Yup it is cloth with clad (bx). In one of my cut-back sitch's I wrapped the cloth with e-tape (it was in _good_ condition -- but I figured with all the handling and squishing to close the box I didn't want it going friable on me. And! I was successful in grounding the outlet (I reused one of those peculiar grommet things). That was exciting and at the same time distressing: Now I realize I probably screwed things up when I cut a bx in another wall in order to add a box to add a couple of outlets via romex 2 years ago before I really understood BX-- those outlets don't show ground, and the box, not only, might be plastic but is buried in a wall. I know where, just not excited about opening up the wall... Maybe, just maybe I've finally learned the lesson of do it right, now, or do it again and again... I'd wanted to remove that BX run then but was eager to get walls up.
A.Einstein: the more ya'know the more ya don't. (paraphrased of course)