Seems to me it has input selection on the front panel for two 4-channel inputs and one 2-channel input. Labelled 4CH-1, 4CH-2 and SFCS and controlled by the "function" rotary selector. Seems also the rear panel has inputs accordingly.
And lots of outputs: 2- and 4-channel rec out, 4-channel output (probably controlled by the volume knob), 2-ch aux output. And a slide switch at the rear upper left corner. And that MCSS connector.... Google was of no help this time.
You are probably right, surteess, the unit is intended to convert a stereo amplifier to a quadraphonic set-up by routing the line out from the existing stereo via the SFCU-2B to a second stereo (power) amplifier. Seems to me the 4-channel out might be connected to both the power amp section of the existing system and the new amplifier, making system volume control possible via the SFCU-2B's "master volume".
I would like to see more documentation on this unit. (And as this is the "2B" I would guess a model "1" would exist, and I think I once saw a picture of that published by JVC_Graz (Klaus).
Regds
Rolv-Karsten