You can see the transition from “fan forward” design to “fan downward” design in the mid 1950’s. I think this brown upright was the first one - maybe 1953 or 1954. As much as I have loved Eureka over the years, I’ll be the first to admit they often copied Hoover designs after the Hoover patent ran out. Wonder when the unique Hoover “fan downward” patent expired.
Hoover had always had a “fan downward” configuration which allowed their uprights to have a sleeker, low profile hood. Eureka’s upright’s were all “fan forward” like a Kirby - up until the early 1950’s when thus cleaning system was put on the market.
So you could easily say Eureka’s classic “fan downward / bag in the back” upright has been around in various forms for over half a century.... and now available under the following names: Sanitaire, Bissell, Perfect, Koblenz, Reliavac, etc.
I think it’s more relevant to group these classic Eurekas as “Dial-a-Nap” uprights from 1970 until now...That dial on the front was a unique Eureka feature for decades.