The plate looks flat with holes. As I said you may need to drill 4 more holes for maximum clearance but that it up to you. You can use Phils rack with the SW Motech kit that allows a Monokey box to be used. Order of install
1. Phils rails.
2 Use closest two sets available holes to seat for the Backrest plate
3 Use the SW kit on a Phils rack and use the farthest set of holes for Phils Rack.
Look at my picture and you will see I mounted the rails, Custom World Backrest plate, and then Phils rack with the SW kit on top. You may need to customize the install a little but if mine worked so should this. As a matter of fact the new plate AST is using looks identical to my Custom World plate. I had to open up the holes a tiny bit to get it perfect. You are better off ordering a CUSTOM WORLD backrest that chancing anything with AST
You can see the threaded hole here your closest set of holes on the backrest plate will bolt to, the backrest plate goes below Phils rack and on top of his rails. It is not installed in this photo but you can see where it will
Here is the new plate that looks just like the Custom world but the old should work if you have it
Here is the three parts together. Phils rails to bike mounts, Custom Worls backrest plate, then Phils monokey rack setup
The backrest without the box attached