The front cottered right hand crank arm and sprocket were usually 40 teeth and the rear hub sprocket could be a 16, 17 or 18 teeth. Gearing would be a Sturmey-Archer 3 speed with indicator chain going into the rear hub. You might be describing a device that fit around the axle with a nut on the back that allowed the wheel to be centered correctly in the rear part of the frame. You have a good solid bike.