K3S (and K3) 2.1 kHz Roofing Filter, Kit*
There are 5 filter positions per K3S and per KRX3A. The 2.7 (or 2.8) kHz SSB filter uses one of these positions.
* On SSB the K3S must transmit through the widest standard SSB filter (either 2.7 or 2.8 kHz). (You may receive on any filter.) All K3S radios must have at least a 2.7 or 2.8 kHz SSB filter installed. (2.7 kHz is included with the base K3S.) For ESSB mode, TX can be set to use the 6 kHz or 15 kHz filters (if installed) and b/w can then be set by the DSP.