These are the new style flat "torsional" type mechanical filters (not the old "disk & wire" loaf of bread) made by Rockwell-Collins, same great specs, more modern design. These are all 455 KHz center frequency and are symmetrical (CW) style filters. I have on this listing 500 Hz narrow bandwidth for CW filters. The model numbers are 526-8610-020 for the 500 Hz . You are buying one filter at a time!!    These are all new old stock from Watkins Johnson and were used in receivers like the WJ-8700, etc. I will ship in a box with the filter wrapped up carefully. Have fun!