Collins made a mechanical filter kit to replace this AM filter with a resulting 6 Khz. Stock AM was a pair of IF transformers that provided a nominal 5 Khz -6db bandwidth but with a very poor shape factor. Without these filters in place, the receiver won't operate in SSB and CW modes. Different SSB bandwidths could be specified with 2.4, 2.75, and 3.1 choices stock CW was 800 hz bandwidth with 300 hz bandwidth optional. In stock form, the 51S1 should have a pair of SSB filters (USB and LSB) along with a CW filter mounted under the filter cover as FL2,3, and 4 respectively.