It may be the cover, but it's certainly not identical to above image of j20. The mystery cover has 4 big sawteeth, while the one on J-20 image above has 8 small sawteeth. Providing we're looking at the same end. if could, of course be that the other end of the J20's cover does have only 4 big sawteeth. One could also argue that the mystery cover has a more pronounced curve, but that's harder to prove.
Edit: the above image of J20 doesn't feature the same j20 that many other recent j20 images do. It seems there was a redesing of the teeth at some point. (the above image could be of the prototype?). Anyway, on more recent images the chute cover does seem to have four teeth on both ends.