Jim,
It is a "backboard" to use along with the extension platform I designed. See Photo. The extension adds about 18 inches to the front of the slip scoop floor. It is for carrying large objects that won't fit in the standard scoop. I use it to carry garbage cans, boxes, tools, trimmers, walk-behind mower, lumber, limbs, etc. You can see in the photo that the plate is held in place by a single pin on each side that allows it to pivot upward if the scoop is tilted down. There is a bar across the bottom that rests against the front edge of the scoop to serve as a mechanical stop.
The backboard is easily removed, but I just leave it in place all of the time since it doesn't get in the way. When scooping and moving mulch, I tie a wire mesh screen across the arms. This allows me to get a much larger load of the light loose material.
JackIL