All my Kodak Retina folders date from the 1940s and '50s, and all produce excellent images. The Retina I in the rear has an Ektar lens made in the U.S. The IA model in the back-left has the Xenar lens, which I believe is a Tessar clone. The three to the front have the six-element Xenon lens.Kodak acquired the Nagel Camerawerk in Stuttgart in 1931. The first Retina model was brought out there in 1934, along with the 35mm cassette which popularized the format. The last folding models of the Retina line were produced in 1960, but the line continued with fixed-lens models until 1969.
There is a good description of the whole Retina line and its history at Wikipedia.