Leitz waist level viewfinder.
AUFSU
An early production accessory, the AUFSU is a multi function viewfinder for the 50mm (5cm) focal length. When mounted normally it can be used as a waist level viewfinder, however due to its extra mounting shoe can be mounted on its side then becoming a covert 90degree right angle viewfinder such as the WINTO or WINKO.
the AUFSU pictured here is the second, later model that has both the secondary cold shoe mount and includes the cross hairs for lining up the viewfinder correctly.
Due to their early production they are fairly scarce, more so when they have their original Leitz box. They are still useful, as they are clear and don't have the tendency to flare in bright sunlight.
A view through the right angle view finder (showing cross hair) and the unit mounted on an MP with Elmar 5cm for size comparison.