I believe the early Corsair came with a 4 pole 9Mhz 2.4 Khz filter, and the 220 8 pole was offered as an upgrade option.. And then became the standard 9Mhz 2.4 filter, right up through the OMNI VI+.. "TEN TEC forever es RTTY TOO" '73, Harry N. Coates Comstock Park, MI WA8HC at charter.net