Required by US law sometime after 1970. Our '68 Ford wagon had separate lap and shoulder belts, but the lap belts were retractable, I think (long time ago, eh?) In 69 or 70, maybe 71, the requirement was retractable belts, and after 72 they had to be three-point.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!