Looks like a casting flaw due to the mold itself. Maybe the box the mold was in had two boards with a gap between them. Sand that should have been compressed by the pattern got pushed through the gap instead, leaving a void where that line is. If the mold was in a rectangular box, the pattern's handle would have pointed toward a corner in order to get away with using the smallest box possible, leading to the funny angle of that line on the skillet.
Not saying with any certainty that that's what happened, but it fits.