That long after the fill tube had been redesigned, and the bag collar changed. Generic companies still produced the old style rounded bottom bag collar on the generic off brand type A bags, companies like K Mart, HomeCare, Blue Lusture, DVC, Sears. Even still today when you buy a pack of Envirocare or DVC Microlined Type A bags, they have the rounded bottom collar. I wonder if it has to do with Hoovers copyright protection, and the fact that that was the only collar template that could be procured. I have cut many of the genuine Hoover collars from the 90s, so I am fine these days with using a DVC Microlined bag for the simple fact of not having to cut down the collar. After all I do have 3 Convertible's that have the early fill tube.
Since I am om my PC, I will include the original 1977 ad for the U4103 Estate showing the new "quick and clean" bag change feature.