Toronto. Leitz is a perfectionist company. The original screw mount and bayonet mount rangefinder cameras used a rangefinder (RF) focussing mechanism. This RF was most accurate for 13.5cm lenses and shorter.
In 1935, Leitz introduced the Telyt 20cm lens and a mirror box called PLOOT for focussing. Using the PLOOT and the Telyt made the tiny Leica into an SLR. Post war the PLOOT was replaced by a Visoflex – later called Visoflex I. This mirror box was followed by the more compact designs of the Visoflex II, IIa, and III. All Visoflexes were made to fit the new bayonet mount M-series Leicas. Visoflex I and II could be bought to take either the screw mount or bayonet mount cameras.
Other manufacturers also made mirror boxes, some for their own cameras and some for the Leica.