I had to purchase another seatbelt for my 300D the other day, it's a long story of how I arrived to that route. While getting a quote from the dealer (I didn't end up ordering from them), I saw on the EPC that the change from the all metal plate to the metal/plastic plates on the seatbelts took place during the 1983 model year, so it's normal for some 83s to have the old style belts, and for some to have the newer style belts.

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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.