The 15x5 wheels would be used on non-wagon, non Wide Oval tire equipped cars.
Tire size using the old designation from 1967 would have been 8.15-15.


15x6 would have been used on wagons with 8.45-15 tires or on non wagons with G70-15 tires.

You also could have specified the 6" wide rim on cars that would have had the 5" wide rims by default by specifying RPO P42 (15x6). RPO P12 was an upgrade to 14x6 on drum brake cars that would have used 14x5 wheels by default.