The Retina I is the smallest and simplest of my Retina cameras. It has a reliable Compur-Rapid shutter and an excellent 3.5/50 Kodak Ektar lens.
My Retina I is a post war model. There are many small variations in the viewfinder types, but the features are essentially the same as the first one produced in 1934 which was designed to accommodate the 35mm cartridge developed by August Nagel. The history of the long Retina line is thoroughly presented in the Kodak Retina Wikipedia page.