I've always thought these were a great solution to a small kitchen. The dishwasher could be installed under a SS sink, taking up a combined width of 36". It required a shallow (6") sink with a rear drain, suitable for a disposer. Like John said, this would usually be a better solution than an 18" dishwasher.
GE liked to claim a focus on the compact (spacemaker) appliances, but I'm not sure how space-saving they actually proved to be. The Mark 27 is a perfect example. On the surface it does save space, but when you stop to consider that, unless the builder was using totally custom cabinets, the 27" drop-in would still consume 30" of base cabinet space, without the benefit of a storage drawer; not such a great idea maybe. And I'm not sure anyone actually manufactured a 27" wide range hood, so again, you'd be forced to use 30" of wall space.
This dishwasher was a pretty good idea for a small kitchen, especially during the 1960's.
lawrence