i dunno but no one ever worries about it and ive seen over two tons of weight put in a 1/2 ton chevy with stock 15" steels and the wheels never broke, they did just fine, so i really wouldnt worry about it..
plus t would depend on which wheels, thjere were probably ten different stock factory steel wheels available for trucks in a given year, wagons, rallies, etc so forth,, im sure they all would have different load ratings,
but again.,,., who worries about it ?
good luck