18lb capacity Whirlpool and Kenmore models were a "special" model when first introduced in the 1960s. There was only one, or maybe two choices, whereas the "mainstream" lines had many model choices.
They went into the mainstream line in the early 1970s. Whirlpool for example offered both standard and super capacity models of each machine. Standard capacity for example would be 5700, 6300, 7700, 8700, etc. Super capacity would be 5800, 6380, 7800, 8800, etc.