'How can you be sure? The 425 looks identical to the 575
Models 575 and 425 are two entirely different cleaners - as distict from each other as, for example, a Kirby Legend II and a Kirby G3.
The chassis is different. The handle bail is different. The handle release mechanism is different. The motor is different. The impeller is different. The Agitator is different. The finish is different. The designs and styles of the original bag would have been different.
Hoover designed the 425 to be lighter and cheaper than anything which had come before it. The chassis was totally redesigned; it was unique to the 425 (the 450, which replaced it, used yet another design). The tool converter was subsequently unique for the 425. The motor was mounted centrally. The chassis also had a highly polished finish, whereas the 575 was the last of the popular-priced Hoovers to have an unpolished finish.
One of the most obvious differences between the two machines is the motor; the 575 uses the larger, heavier, more expensive motor - very similar to Model 700s, sealed and without the cooling fan, but slightly more powerful.
The 425 introduced a brand new, lightweight motor. It was air-cooled by an internal fan on the armature, and had a much smaller impeller. The example shown below is actually a pre-restoration pic of my Model 300, but apart from the built-in headlight on the 300, it's a very similar motor. As you can see, it's a separate unit underneath an outer hood, whereas with the older style of motor, there is no outer casing, just a cap held on with 2 screws.
