It's important to consider individual characteristics like the triangular area as just a small part of making an identification of an unmarked piece. While many Wagner skillets, marked and unmarked, have that flattened triangle at the base of the handle, it's not necessarily exclusive to that maker.
Even BSR skillets, one of whose main identifying characteristics is a ridge that does not flatten where it meets the sidewall of the pan, are sometimes seen with a flattened area there. But it is other characteristics, like a heat ring or the size and placement of size markings, that let us know it's a BSR.