What is the headstamp?
Molten brass hitting the web of the case?
Now, take a picture of a sectioned .40 S&W case and see the difference in web thickness/length.
That was probably a lap formed when the sheet of brass the blanks were punched out of was originally rolled at the mill. Used to see things like that on a larger scale in steel plates used in shipbuilding. Manufacturing processes and quality control have gotten so good that rolling mill defects are seldom seen by the final consumer. If that had been on the outside it would have been rejected before the case was ever finished the drawing process.