Double C Channel - Shell Off and Going Back On

[Aopsahl7]

All,

I am restoring a 74 ambassador. So far I have removed all interior and skins/insulation etc. Today, I hoisted the shell off and pulled the trailer out. Tomorrow I will start on trailer restoration.

I am planning ahead and thinking about when I go to put the shell back on the trailer. With a new sub floor, insulation etc, the only part I cannot fully wrap my head around is why there are a few sections of the shell that have a second "C" channel below the to wrap around the edge of the plywood subfloor. Most of the rest of the shell simply screws and bolts (through the frame in some instances) from the shell into the subfloor.

So with all of that out there it raises the question of why and do I need these sections of lower C Channel? Also, assuming I do need them, how do you get them back on the subfloor considering my new subfloor will already be installed on the trailer?

Few pictures I stole off the internet to make sure I am clear.