The A-138m is a 4x4 mix matrix where you can mix up to four incoming signals in all possible variations to four outgoing switches. The Signals can be audio and/or voltage controls.
For each column of the A-138m you can chose whether the potentiometers are uni-polar and thus work as attenuators or if they work in a bi-polar mode, turning into polarizers.
Setting a jumper determines if the potentiometers of the topmost row generate an adjustable DC voltage (working as offset generators) if no cable is inserted into the upper input socket. The function is identical to A-138a/b.