what was the last year for just lap belts?

My Mom's 1967 Coronet did not have optional shoulder belt. My 1968 Dodge Dart had the optional shoulder belt with the same buckle as the lap belt. Now my 1972 Dodge Dart has the optional shoulder belt that hooks on a post on the lap belt. I use the shoulder belt when on the Highway, but it makes it impossible to reach the vent door with it on.