Generally Multiplex Adapters were designed to be a "UNIVERSAL" adapter. Meaning if it needed an adapter, you could use any of them out there. By the 1964 model year, anything that had FM, usually had FM Multiplex Stereo. So they were fairly shortlived ,1960 to 1964 as an outboard adapter.
I know FISHER made 1 or two models right at the beginning of Multiplex before the FCC had made their decision as to which system would be used. Although FISHER lost $$ on the MPX-20 (the crosby system multiplexer unit for the R202-T) they made it up with the sales of the MPX-100 and MPX-200.