1653 is your most likely answer.

Most of the Sherwood Receivers in CCF files at RC use code sets roughly matching Audio/0653 (but the 9960 doesn't include that).

If your receiver is one of those, then setup codes Audio/0491 (another the 9960 doesn't have) and Audio/1653 (which the 9960 does have) are close relatives of 0653.

Another Sherwood receiver in a CCF file at RC uses a setup code totally unrelated to Audio/0653. If you have that code set, you would find some of the functions in CD/0461 would work. But the 9960 doesn't include that, so even testing it would require loading it over the phone.

I don't have info saying any Sherwood Receiver uses Audio/0502 (which the 9960 doesn't include) but Edmund probably knows something about that, which I don't.