Super_L fails to bind Win key in .ratpoisonrc

Following the example in the docs ~/.ratpoisonrc # Setting the escape key to the Windows key escape Super_L works fine on one machine. On another however it refers to the Fn key. Both are laptops by the same manufacturer. One difference is one has the Win key to the right and Fn to the left while the other has them swapped. How to use the Win key on the second laptop instead of Fn?