My 1980 300SD doesn't have the rack dampner bolt either. I have a 79 300SD That does. What I suspect is that the orginal pump was replaced with one from a non turbo engine or at least the back plate.

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1980 300SD Manganese Brown Metallic Clearcoat 248k
1979 300SD White 148 k
1984 300D Silver project car for daughter
1982 300D wrecked
1980 300SD 310K not running