I'm not sure what it was, but you are correct that only the preproduction models had that little circle. If you have a case that intentionally accommodates that circle, I wouldn't be to dismayed. There is nothing in that particular location that needs protecting, so that isn't a concern. In fact, the more open back should allow for more sound to make it to the back mic and also more sound to escape from the speaker. Additionally it could help with heat dispersion as opposed to a case with a more closed back.