GIG,

They are compatible, I have the struts from a 1999 model on my 2013 S40.

You will need to get the fasteners from the 1997 model to make it fit correctly.

There is a slight difference in width between the rear fender on the sissy bar model and the later model S40s. Cover the sides of the rear fender  with thin cardboard before you attempt to slide the entire unit into place. I tried mounting the struts and then sliding the sissy bar into the struts and I couldn't get it to work. After assembling the struts and sissy bar when you attempt to slide the unit into place you will be able to see the difference in width. I had to spread the back end of the unit, with a spreading clamp, to get clearance to slide the unit into place.

Kenny G