What is the purpose of the groove behind a TR3A door. I am putting on new interior panels and this groove baffles me. There are three tabs cut out (only two in the picture) which look like they fold over the groove to hold something in. I tried putting the fur flex in the groove but that doesn't work. When I put the panel behind the door on, there is a gap that seems like it needs something else to fill it in. I am working on one of those projects where I didn't take it apart. What else goes here ?