Shadow Zomb is the 3AA exclusive variant that was released in conjunction with the Boiler Zomb, which was the first figure release in the Adventure Kartel toy line. He came with the same severed Bramble head that Boiler Zomb came with. While the standard Zomb has a "B" on the back of his boiler-suit, Shadow Zomb has an "S."
Action Portable Shadow Zomb
Shadow Zomb was also a 3AA exclusive release during the AP Zombs sale.