The canvas at the rear of the walkersIJ It's my understanding it's part of the corn chopping kit. They seem to work OK without it. Controls the flow to or return from the chopper inside the rear hood.
The purpose of the curtain in the back of the walker hood is to protect the ends of the walkers and the top of the hood from corn cob damage or any other incedental hard object going thru the straw chopper.