Here is some parallel information. A couple of years ago, my electric utility upgraded to remote meter reading. I knew the then chief engineer of the utility and he gave me a thumbnail description of how the system worked. It modulates the AC zero crossings digitally. And so, all my old digital clocks started running fast because there were now additional zero crossings. I started opening them up, and discovered that the RC low pass filter shown on the chip data sheets was absent in every bloody one of them. Even though I found 3 different clock chips, none were in circuits recommended by the chip designers. All different makes and models of clocks, including some clock radios. Adding the recommended resistor and capacitor fixed the problem. My CM 15 runs fast for this same reason, and I will eventually get in and filter the optical coupler output which provides the 60 Hz signal to the CY7C63723 chip.
The real problem which arises here is that any filter added externally to the CM15 will also remove the output signal. I may end up bringing the output out on a separate set of conductors, plug the CM15 into a UPS, and feed the output directly into a power outlet.