What Ruud is refering to is parallax correction. The lines or boxframe in the viewfinder matched the focal length of the lens and the angle of the lens used, as in the Leica. When you changed lenses the framing in the viewfinder changed to accommodate the angle covered by the particular lens fixed to the body.
Other cameras like the Zeiss , Voigtländer etc.,came out with fixed lenses in which case the framing in the viewfinder also remained fixed. There are many new models on the market even now as we speak, and they are simply great, often oudoing their digital cousins. I recently managed to find a Kodak Retina 1b on the street in Bremen , Germany. It is NOT a Kodak but a camera manufactured in Stuttgart, Germany between 1954 and 1958. It also just so happens to be the same model used by Edmund Hilary to record the conquest of Mt. Everest in 1955. The other camera used was a TLR Rollei. As soon as I return to my home country at the end of November, I will post some of the results from theRetina.