300D is a W123 chassis. The (81 and later) 300SD is a W126. Different car with the same engine and transmission. The 126 is a little larger and has a bit more complexity of electronics. The 78-80 300SD was the W116 chassis. This was the predecessor of the W126.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.