A very nice five oscillator instrument. This instrument includes the 904B high pass module and the 904C filter coupler. The filter coupler allows one to create notch and band pass responses by coupling the 904A and 904B - too much trouble if you ask me, and too much panel space taken up by dubious sonic capability. I suspect that the 984 matrix mixer was developed specifically to be a low cost panel space filler. It fills six panel spaces with knobs while requiring only simple and low cost electronics (even by 1960’s standards) behind the panel. In contrast, more useful modules such as oscillators or filters were difficult to build and therefore expensive.