Looking thru some papers I've kept over the 45 years I've had my -6. Found the install sheet for a set of Koni shocks I put on the rears not long after I bought the car in the late '70s. They are called Special D and the damping effect can be adjusted (I've never done it). Wondering if anyone else ever tried them.

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Nope. I've never liked Koni products. Are they still in business?

I MUCH prefer the old school adjustable Koni oil shock. Porsche has had them made up in black for factory replacements. NOT cheap, about $600 for a rear pair.