This is the detachable wire channel (shield) located on the back wall inside the grill. The wires for the front grate probe (#26) are encased under this when installed to protect the wires from damage. This part is held in place by lining it up with the tabs on the inside back wall of the grill. These tabs have holes in them, so once the shield is in place you can used the small cotter pins to hold this part in place. There are two of these channel/shields in each grill. This is the longer 11-1/4″ one.
In the manual for the Oklahoma Joe’s Rider DLX Pellet Grill 900 and 1200 models this is listed in the parts key as item # #27.