From what I remember from working in a bike shop in the early '70s, Suttons began showing up in the late '60s, pre-bike boom. Importers were bringing low-end Japanese bikes to compete with the relatively few 10-speed bikes being sold in the U.S.

The bluntly cut, thick-wall lugs are a hallmark of those Suttons and equivalent Japanese bikes. Another is the top tube length, which was the same for the various frame sizes of the lowest-end models. Cheaper to use parallel seat and head tube angles and simply cut those tubes to length accordingly for the taller and shorter sizes.