The 1981 300SD can use a Delco GM or R-4 compressor. I think ND compressors were for W126 models and other benzes after 85.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.