Nice Eureka Steven...
I bet you are right about the bag....the pattern looks right for the vac, but it should have a plastic decal or "stick-on" that is a vertical version of the brandname on the Dial-A-Nap nameplate. Some of the lower end Dial-A-Naps had the name printed on the bag, but others had a nice shiny name tag glued on......
If you see some adhesive or remnants of a vertical rectangle on the bag, then the name tag has fallen off. This could also just be a generic Eureka replacement bag from just a few years after you were born - mid 1970's, I'd say. But again the design of the bag is almost indentical to the original.