That's a piece of leather. It gives a bit more stability than the existing front rest. I've got a new Protektor front bag but I've got to figure out how to fill it as there is not much room to get the sand in. I think I'll make a funnel out of some thin metal.
I cut a section of brass out of a .308 casing and made a nice tube to hold the nozzle open, then a small funnel into that. Use a wooden dowel to pack the sand in there.