I was a bench tech at a Muzak franchise, and saw many different SCA receivers, but never one of this brand. It looks like a McMartin clone, and receives one FM station using a crystal controlled oscillator, and a front end tuned to that specific station. The main channel is amplified by two or three IF amps, usually two limiters, a Foster-Seely discriminator, not a ratio detector, then the un-de-emphasized signal is fed into a 67 kHz tuned coil or transformer to trap the sub-carrier signal, which is then converted to 455 kHz, and amplified using two IF amps, two limiters, and another discriminator. A type of filter/amp system has also been used, notably by Browning Labs. The audio would have had about 8kHz bandwidth, not HiFi, but enough for the purpose. The main channel is usually of very high quality, as that's whats necessary to accomplish the bandwidth needed to amplify the sub-carrier signal.
You're right about the outboard multiplex unit, as there aren't enough tubes on the chassis to indicate one on-board, unless this one is a real cheap set. You could re-cap it and see what station might or might not come in, but maybe it would be better to just keep it a an artifact of broadcasting history, as the crystal needs to be changed to change stations.
Hope this helps. Anyone else??