Mine had two welded in nuts right at the base of the driveshaft tunnel near the back of the seat. I used those two but had to weld in two more near the outer rear edge of the back seat. I used a universal retractable three point system by anchoring the retractor on the hat shelf.
dean

As the 544 crowd hasn't yet jumped in with a more definitive answer here, I'll mention that I don't recall my '66 122S (Canadian market) having attachment points for rear seat belts as I similarly wanted to add them. So an earlier 544 almost certainly wouldn't. If you intend to retrofit lap belts you could use a re-inforcing plate on the underside of the chassis, something preferably a bit bigger than a fender washer. It of course wouldn't be tested to DOT standards and you would have to carefully consider whether a buckled frame in an accident could do more harm than good to a rear occupant and in addition not having shoulder belts to meet more modern reqirements.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now