Magnavox produced Stereo/Radio/Phonograph equipment which had a connection for a FM Multiplex unit. The purpose of the FM Multiplex is to reproduce the FM stereo broadcast. Dealers could install a FM Multiplex unit or the homeowner could DIY. It wasn't complicated. One known adapter kit is the KO 597B. The following is a list of model numbers in which you could have an adapter kit installed. The list may not be complete. Click on picture to enlarge.