Found a Ring that says FW1 with a skull in the shape of a fist-What is it?

I found this ring hidden in a box at my deceased grandfather's house. He was a WW2 German Navy Sergeant. His son, my father was in the 101st air born in Vietnam. My dad was a biker and it looks like a ring he would have worn. (he is also gone) It is a ring in the shape of a fist punching outward. Each knuckle on the fist has a symbol on it in this order. A German Cross, FW, a skull, and the number 1 with the thumb jammed underneath the fingers. What is this thing?