The wood shown on the side is the floor. When the M577 sets up in a static command post mode, the tent is erected over the lowered ramp and that wooden floor was dropped underneath. It is not a standard item. Some units use plywood as a floor, others just use a canvas tarp. Depends on what's available and weather/terrain. Sometimes nothing is used.
Yes, there are plenty of things I do not know. Yes, the trim vane is made of plywood. I broke one in my early years. I had lowered the trim vane to check inside the engine hatch and forgot to raise it back up before moving out. Cracked that thing like a twig.
Just a big piece of plywood stuffed with some styrofoam. You can see the yellow styrofoam peeking through the paint where the engine hatch handle rubs against it:
