Well, I'd say what makes it "rare" is that the 603 was only available in the 124 for one year, whereas the 123/617 combo was available for 9 (?) years. So right there it's going to be a lot less of them. I can't remember the exact number produced for our market, but it isn't a whopping total. I wouldn't call them rare, but I'd call them rarer. The percent left on the roads is probably pretty high (like with many old diesel models), because they're nice cars and people tend to hang on to them.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)