In the late 50s Fisher prepared you for every conceivable form of stereophonic broadcasting the 1960s might throw your way. With the selector switch set to "Radio" (note the clever use of the little triangle symbol to denote stereophonic sources) you could connect an external AM tuner to the "B" tuner jack and listen to FM-AM stereocasts. Or you could uses a second FM tuner connected to the same jack for the much better sounding but much harder to find FM-FM stereocasts.
For those willing to try to predict the FCC's anticipated ruling on multiplex broadcasting, Fisher also included a set of MPX jacks on the "top" of the chassis. (They're actually on the side when installed in a console.) With Fisher's recommended optional MPX-10 connected you'd be all set to receive the experimental Crosby stereo transmissions from WBAL, NYC; KDKA, Pittsburgh, PA or WJER, Wilmington, DE. Or, if you weren't quite so adventurous and were willing to wait until after April 1961, you could buy an FCC approved GE/Zenith system MPX adapter.
The nice thing about the 610-ST is that if you bought one of the nicer MPX adapters equipped with feed through connections, you could run a cable from the FM OUTPUT on the 610 to the Channel "B" Aux input on your MPX adapter and still leave your external AM or FM tuner hooked up to the "B" tuner input. That way you just had to set the selector switch to the "triangle" Radio position for either FM-AM stereocasts or multiplex stereo broadcasts and didn't have to redo all your console cabling. Pretty nifty back then but not terribly useful these days.