Comment by rhinoceraptor

The linked manual site was useful, I have four various Leica III series cameras that were my great-grandfathers. One of the more obscure pieces of the collection that I'd never seen before is the Visoflex [1], which essentially transforms them into an SLR.

Rangefinders have a fixed viewfinder for framing, as well as a special focusing viewfinder, which projects two images superimposed from different angles, via a very small mirror and beam splitter. The mirror is mechanically linked to the lens' focus ring, and when the two images align in the viewfinder, the lens will be in focus.

1: https://www.cameramanuals.org/leica_pdf/leitz_visoflex_ii.pd...