What is the "base" and "mount" here

Really great work! But I'm confused that why the "base" and "mount" need extra two ports? There are only two ports(left arm and right arm) in the original XLeRobot configuration. 

Also, is it compatible to the new version(dual wheel)? Which is really important for me to replicate! And does it support the motion mapping from HMD to head camera? :)