Hoover 300,
First of all, thank you for all the photos and your research of the early period of Eureka floor care products. I also appreciate the credit you gave me for the advertisements.
I had found the Model H Junior in other advertisements but not the Model J, so your photo's are expanding our knowledge of those rare cleaners.
Secondly, I checked your top model list in reply #29 and was unable to find any ad's for them likely due to their short production runs. As for the 'D-174' I don't think it existed. I believe you got that from the 1958 Shipping Record and had thought the same thing until I located several references of the Model 'D-171'. I also wanted to mention that I believe the main difference between the D-171 and D-272 is that the latter was made to be compatible with the new Disturbulator.
A couple suggestions for you are to check with 'DadsVacuum', who posted the 1958 Shipping Record. I noticed that he didn't post the page for the tank cleaners, hand vacs and the auto vacs; so his Dad's list would at least have the production periods in them. The other possible resource is paying a visit sometime to the McLean County Museum of History that houses Eureka's archives.
Thanks again for sharing all your research.