In the assembly shown below, I need to constrain the black part (Shuttle) shoulder within the slot of the silver part (Back Plate).  I applied a slot mate, but as you can see in the second screen shot, Solidworks interprets the black part as though it is a female slot rather than a male shoulder component.  Hence, when you rotate the Shuttle, it moves until it is stopped by a theoretical bolt inserted in the two slot shapes would limit further movement. 

Can any one advise as to how to constrain these two parts properly?  I've tried a width mate, but Solidworks tells me the face at the end of the Shuttle shoulder and slot face are not valid selections.  I guess I could cut a flat surface at the very ends of the shoulder and on the insides of the slot ends, but that seems like a less than elegant solution.  Part files attached in case anyone wants to try and solve this.  Thanks!