Thanks for posting that, Patrick!
It would suggest that Eureka's tapered hood style—first used on the Model 266—was introduced around 1965. It's also interesting that the term "Imperial" was used to denote the full-featured model, and that its color is cream versus the deluxe model's ivory. (From Pinterest: "The main difference between ivory color and cream color are in the yellow undertones of both hues. When comparing ivory vs cream, the cream color will reflect its namesakes creamy yellow undertones that are found in butter or thick dairy products. Ivory color will be a brighter white.") It's possible, then, that the Eureka-branded cleaners used the same colors of distinction.
Another implication is that Sanitaire's ST bags may have been so named for "Sateen Twill".
Thanks again.