I haven't seen Bilsteins for Volvos, however every other strut car I have worked on that used Bilsteins has the shock fitted upside down in the strut. This means the piston seal is actually inside the strut body away from any dust/dirt anyhow. The chrome wiping surface you see through the springs is in fact the outside of the shock. There is a wiper that keeps excess dirt out of the strut cavity. I have never had a problem with this setup and the wipers last a long time without failing. That could be your answer since I haven't ever seen dust covers for this setup.
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