No the rear cells are not interchangable. Ektar was/is a Kodak trade name not a design. Though the name did denote a quality. They were Kodak's quality lenses at a time when Kodak produced some of the worlds best Large format lenses. I do not anticipate you having any trouble mounting those lenses to lens boards. The 203mm is a 4 element Dialyte design. The 127mm lens is to a Tessar design . Both fine lenses.