There were 3 of these same "round head" models in
they were the 305, the 27, and the 28. (The 26 had a roundish head, too but was not identical, except for color, like these 3). The post that Charles Richard put up of the 27 pamphlet is great, you can see how much it looks like the other two, except it had a rather different multi-color motor cover, base, and bag.
The 305 (which was built primarily before WWII started in December of 1941) had some major differences from the 27 and 28. For one thing, it did not have side conversion for the tools. You would have to actually take the bottom plate and belt off the cleaner and attach the converter over the suction opening in the bottom, much like many of the Hoovers of the 20's and 30's. Not very convenient, I'm sure. If you look at the 5th picture of these guys above - the one where you can the the right sides of the 3 machines - you will notice that the 305 does not have the opening in the side of the base in which you would slide in the attachment converter. Another thing about the 305, one of its most famous, is that the word "Hoover" is actually molded into the top of the motor hood - very cool. I have heard over the years that the 305's tended to have a lot of bearing problems and that is why you see very few of them left today (in addition to being around 70 years old), although they had a fairly decent production run over all. My 305 runs great, although it is definitely a bit louder than the 28. The Model 27 had a very short run right after the war (45-46), and then was followed by the 28 which had one of the longest and more produced runs of any of Hoover's individual models. Hope that answers some of your questions Chris!