Sunbeam model and service numbers are located where the beaters plug in or where the mixers attach to their stands. Antique and vintage kitchenware reference books and websites are another source that can identify a model.
DecoDan.com, a vintage appliance reconditioning and repair company, notes that the M4 was the first Mixmaster sold to the pubic. The supplier was the Chicago Flexible Shaft Co., which sold the product around 1931. Ivar Jepson, also known as "Mr. Sunbeam," had gone to work for the company six years prior, as reported by The Toaster Museum Foundation.
Like model M4, model M4C was made of cast iron. It was manufactured from May through October 1931. Although this model didn't have a decal, model M4F had one located on its motor housing. M4F was the last model to have a juicer bowl that screwed onto the mixer head, stated by DecoDan.
Model M4H, which was manufactured during the summer of 1932, had an oil dropper and pulp screen, and other attachments were available for it. The next year, a three-speed version, model M4J, was introduced. The updated version, model M4K, was manufactured from 1933 through mid-1935.
By May 1935, when Model 1 was introduced, Mixmaster was fast becoming a staple in American kitchens. With 10 speeds and 120 watts of power, Model 1 signified Mixmaster's entry into the modern era.