In the 1930s and 1940s, more than one individual filed for and received a patent for what are termed combination cookers. Combo cookers, as they are also known, were multi-purpose utensils designed to, separately and in combination, cook multiple foods simultaneously.
Typically, combo cookers consist of three pieces: a deep pot, a shallow pot which fits into the deep pot, and a cover which fits either.
John C. Johnson
In 1934, John C. Johnson was awarded a patent for a "Combination Cooking Utensil".
USD91487 (1934) - Combination Cooking Utensil (John C. Johnson)
Johnson's cooker featured loop handles on the deep and shallow pots, and its skillet/cover's flat surface allowed for the possibility of stacking another pan on top.
Edward W. Blankenship
In 1940, Edward W. Blankenship was awarded a patent for a "Combination Cooking Vessel".
US2199687 (1940) - Combination Cooking Vessel (Edward W. Blankenship)
Blankenship's cooker featured loop handles on the deep pot. Its shallow pot had a handle and proportions more like a deep skillet. Like Johnsons's design, its skillet/cover's flat surface allowed for the possibility of stacking another pan on top.
Projections on all three pieces served as pour lips, but, when rotated out of alignment with each other, functioned as vents. Blankenship's patent described these as both functional and safety features.
Other cookers of similar design exist, but their provenance is uncertain. They also often serve as points of confusion, as their parts sometimes fit reasonably well with those of other makers.
One such often seen is described as the "Tab & Notch" combo cooker. Its pieces each have tabs on and grooves in the rims which engage each other when the pieces are uniformly aligned. If there is a function to them beyond that, it is unclear. Perhaps the tab and notch arrangement was simply a device to circumvent claims of patent infringement.
Collecting vintage combo cookers can be daunting when all the parts are not found together, so it is best to familiarize yourself with the various types before purchasing any orphaned parts.