I don't think there was any moment in time where they "switched" from low to super high shank. In collecting vintage Kenmores, I've never found any rhyme or reason for which machine has which shank type - there are models with SHS that are older than models with low shank, and vice versa. And it's difficult to find any resource that lists all the models and their shank types/features so you can know what you're getting if you bid on a machine sight unseen.