I was looking a my TC again to try to help a guy on FB having problems with his door sticking in the latched open position. That middle piece is definitely the hold open latch. There is a wheel and a couple torx screw heads visible up inside the middle track that match that part very well. So the hex screw must be used to adjust the tension on the roller. Glad I found this info because as you can see I've not gotten much lube there when spraying the tracks. This will be another spot to hit with the better red lube spray I just started using.