Ken,
I love the SP-40N Use & Care Manual. Thanks for posting it. I did not realize until now, how similar this reintroduced GE Portable Dishwasher was, to the other mid 1950's GE dishwashers such as the 1955 SE40M4 (sink combo model) and the 1SU60N (undercounter model). By 1955, GE had modified both the upper and lower racks for the dishwashers produced for these few years in the mid 1950's, as well as the silverware basket.
The 1953 model dishwashers were still being produced with the same basic rack design that had been used since before World War II. This design features the rectangular shaped removable glass tray in the upper rack. The only real change that was made in the design of that rack style, started to occur with some models, in 1950, with rubber coating.
Then in 1954 or 1955, the racks were radically changed. The lower rack was more rounded, with the plates tilted further back. The upper rack also took on a rounded shape, and that change removed any items from the immediate upper center section. The silverware basket was widened, and opened up from the former cup design. After watching several of my dishwashers operate, via Plexiglas lid, I have concluded that the Bakelite Impeller never did sufficiently lift water to the upper center rack area, even with the old style silverware basket removed. I do not know why GE waited so many years to figure this out on their own. They most assuredly had a huge customer dissatisfaction with the cleanliness of glasses in the center section of that upper rack.
In 1957, with the introduction of the next generation MobileMaid models such as the SP40S1 and comparable static models, the racks were changed again - back to a square shape. The upper rack in this design had a slight tilt built in, and the lower rack moved the area for large plates, to each of the four corners. Also, the silverware basket was moved to the upper front area. And we also have the introduction now of the metal bowtie shaped impeller. This, in my opinion, was GE's best improvement to dishwashing, which lasted until the introduction of the spray arm technology, I believe, in the early 1960's.
Mike