The guideline at the left is not a guarantee that it is fake, but it is a red flag. The stamps at the edges of the sheet often have wider margins that can create a more plausible looking 315 by trimming the perforations off a 304. But the top and bottom margins are still too short and would not pass the minimum measurements that rogdcam posted.
Here is a corner margin copy that looks like it might be big enough to be genuine yet PF (565078) says "It is not Scott 315, rather, it is a Scott 304, with perforations trimmed away at left & bottom."
