Hold it still
Grubb keeps one stamp on a string. He will press it onto your stub if you sign for it. The number, the date and the little mark in the corner all come out of your signature, so two people never get the same stub.
Hand it over. I've got the stamp right here.
Your stub
Blank stub. Nothing on it yet.
Signing costs nothing and moves no funds. The picture is drawn in your browser and never leaves it.
What it's worth
Nothing, on its own. It is a picture of a number. But the same signature that made it is the one Grubb copies into the book, and the book is the thing that gets read out later.