Here is looking more into the dome cavity. Note the parting line is at the wire groove, and how the mold halves come apart. With this "3-piece mold" the two hinged pieces go outward leaving the dome portion which is connected onto the base part. If the mold halves part all the way across the dome you get MLOD or Mold Line Over Dome, and it is referred to a 2-piece mold. The "flash" is extra glass that flows in between the mold halves at the parting line. It's that sharp stuff that usually breaks off. Core detail is here: [id=180525049]