Which Finder ???

[Filzkoeter]

The dotted line is the line for parallax correction at short focus distances 😎

SBOOIs are awesome for shooting 50mm! Using it with both eyes open is great.
I'm hunting for a SBLOO myself. Have a VIOOH, but it's way too bulky, it sits way to high on the camera to be of pleasant use.

 

[Luddite Frank]

One other caution on the Tewe Poly-Focus: they made a version for the Square-format ROBOT camera: 24 x 24 mm. This version is probably not very helpful on a full-frame 35mm.

 

[Erik van Straten]

A nice finder for a IIIc is the WEISU. A very small but effective 35mm finder.

Leica IIIc, WEISU, Summaron 35mm f/3.5, Tmax400.

Erik.