The tenon plate is essential.
#1) It is a spacer between the stanchion and the window frame. The screws that screw the stanchion to the frame are a critical fit for length. If too long they can hit the glass and break it. Omitting the tenon plate could be just enough to cause this problem.
#2) The side screen, if you plan to use them, goes behind the stanchion and closes against the tenon plate. Without the tenon plate, it would just flop around behind the windscreen. Proper fitting of the sidescreen in this area helps to "seal" the car from wind and water. Of course, most people say these cars don't so much leak, as actually invite the water in with you. I've been driving in the rain a fair bit lately, and I' am staying dry in all but the most outrageous downpours.
It is clamped between the stanchion and the window frame by the screws.