The 500 series machines in the ad are 1970 models. They were part of a long line of machines made into late 1973 which used this basic console platform from 1967 or so until '73.
As mentioned by others, these 1970 500 series machines were very popular and common. Probably on the lower end of Sears' "bread and butter" models (the ones they sell a lot of and make the most money) but as mentioned also, there wasn't much else a user needed. Decent capacity, great agitator, lint filter, two speeds, porcelain top and lid...they had the necessities covered.
Mark - If you'd care to see a few of Kenmore's maze filters, I can get you some pictures.
Gordon