Cast iron wheels, with or without ribbed backs, were last used on new cars in the 1950s, and banned from interchange service before 1970. They were the exception even in 1950, and exceedingly rare 20 years later. Most cars migrated to wrought steel wheels long before they were required.
According to this source http://cs.trains.com/mrr/f/88/t/152100.aspx .
Bathtub type coal gons like the Walthers cars are much too new to have ever received wheels with ribbed backs.