I've found lots of pictures of this car in a certain online digital library, but there's no information available there except for a year : 1969.
Digging deeper, I've found two conflicting stories :
- in 1969, Minicars Inc of Goleta, California (near Santa Barbara) designed the Minicar, a 2-door 3-seat small car fitted with an air-cooled six-cylinder engine, automatic transmission and air conditioning. There's no picture in this source.
- in 1969, the University of Pennsylvania's Center for Urban Research and Experiment unveiled the Minicar prototype, designed in cooperation with Minicars Inc of Santa Barbara, California. It was a hybrid car, powered by both an electric engine and a 40 hp flat air-cooled engine. The project started in 1967 and was funded by the US Department of Housing and Urban Development and by the US Department of Transportation. There's a picture in this source : it has roughly the same proportions as the puzzle's car but it looks like a real car, not like a crude prototype. See the attached picture.
So I would answer that the puzzle is an early prototype of the Minicar.