The extender screw is designed for the mini, classic and drifter model. It is a M8 high tensile cap screw specifically designed to connect an extender to the main seat frame. You can also use M8 high tensile screw from your local hardware store; as long as it is M8 high tensile around 20mm long, they will work even if it's not exactly the same head!
Let us know if you have any questions!
It is also the same screw that connects the extender to the t-bar at the other end.