The top side hose port on Eureka uprights was replaced by the Dial-a-Nap height adjustment dial some time in the late 1960’s. So all you need to do is find the year in which the Dial-a-Nap feature was introduced. It could be 1970.
There is a really cool television commercial for the very first Eureka Vanguard with the extra large box-top dust bag which I think is from 1969. That very first Vanguard still had a top-side hose port. I will post a link….
