Leitz 3.5cm viewfinder
External brightline viewfinder
Leica screw mount cameras (LTM) such as the Model IIIg lacked wide angle viewfinders for lens such as 35mm, so external viewfinders like the WEISU (early 1930's) or the SBLOO were required for framing.
The SBLOO is an exceptionally large and bright viewfinder even surpassing the 100% viewfinders in todays professional cameras for clarity.
External optical finder. Pictured above is an Optical Bright-Line viewfinder for 35mm (or 3,5cm) lenses, catalog number for the viewfinder is SBLOO, shown here mounted on a Leica IIIg.