The VA Zombie Killer appears to be a clone of something VFC and has nearly exactly the same ambidextrous mode selector pin-through-the-mechbox and selector gear arrangement as, for example, the VFC KAC PDW. You may need to remove the ambidextrous mode selector dials to slide the mechbox out of the lower.
BTW, this is the company that produces them:
http://shop.ehobbyasia.com/jm-m4-urx-rifle-aeg.html