It's an abbreviation for 'Design & Manufacturing' - they were the company who built dishwashers for a number of appliance companies. They 'grew' out of a company called Avco, a division of American Standard (the plumbing fixture company). They built a dishwasher branded 'American Kitchens' that had a rotating round upper dishrack - this launched the 'Roto-Rack' dishwashers sold by Kenmore, Kelvinator, Admiral and other manufacturers. Later models had standard square upper racks. Other brands D&M built dishwashers for included Tappan, O'Keefe & Merrit, Caloric, Chambers and yes, for one year only, Whirlpool.