When we first designed the Pollywog, we added a hole into the back for spinners to add sand if they wanted to weight the drive wheel to add more momentum. That little black stopper is for placing in the hole after it has been filled with sand. This is a feature that is no longer available on the Pollywog, because additional weight is not required.
If you would like to order a black stopper to fill the hole in your Pollywog, please contact us. Keeping the hole open or plugging it does not effect function whatsoever.