The car had aftermarket lap belts. After decades of using the usual shoulder/lap belts my wife and I did not feel comfortable with only lap belts. I didn’t see a way to use combination shoulder/lap belts because the B pillar is just two window tracks where you would want a strong anchor. And the rear seat covers the place where the retractor would go.
I found these three-point harnesses with a retractor that can be installed behind the rear seat. Once the harness is adjusted the retractor makes it easy to get in and out and also allows movement while driving.