The DW660 mount you posted is for a quick change z axis mount (aka hickwick’s universal) which is not the standard in the MPCNC parts list. I did not go with the hickwick’s mount as my base because I am milling about 95% of the time…no need for quick change. Also, I had rigidity concerns.
The air jet mount is the standard machine-side half of the lower mount modified to aim compressed air at the tool bit when it is cutting. The purpose of the jet is help cool both the chips and the tool bit and to blast the chips off the bit and out of the curf. I designed the jet and the dust shoe to work together. The jet blows the chips toward the dust shoe and the goal is for the dust shoe to suck up the chips.