From the announcement:
Weapon Swaps for the Leviathan Crusader and Mortis
These are Kickstarter exclusives and not likely to be released until the variant that uses these weapons are made into complete model kits! If you want them, now is the time to get them, or you may be waiting a very long time for these optional weapons.
These hot swap weapon arms will allow you to instantly customize your Crusader or Mortis, Twist and remove the current arm and slap on one of these beauties!