My car is an 82 240D, 123. Since your 83 300SD is a 126?...the question would be, would the 126 and 123 latches and belts be the same. The pictures on ebay looked like the same part? I'm not familiar with the 420, is that a 124?

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold