This is a very rare and usually hard to find 2x converter by Carl Zeiss Jena.
I think it is a 4 lens design but I´m not 100% sure. I tested the optical performance of this converter in combination with the Carl Zeiss Jena DDR MC Flektogon 2.4/35. I was positively surprised by the quality which is quite good when the lens is stopped down to f/5.6 (turning the effective aperture into f/11). Of course, there is some loss in sharpness and contrast but less than I expected.
This converter is very small. It´s outer diameter doesn´t even fully cover the lens mount of the Mamiya NC series cameras.