Hey, I ordered a few blank 801 carts elsewhere in advance of receiving my Prodigy to try them out, but they are a COMPLETE mystery to me. There is a black whistle tip and a white cover over it... but where is the stuffing? Is the white cover so complex for some reason I am missing? Am I missing something that I have to buy filled carts to get? I am SERIOUSLY completely MYSTIFIED by this! I've never used an 801. I've searched here and found NOTHING... I feel VERY alone! 