The last four digits are date code, year, week of year. F455FA21 is the standard filter for 75S3-B/C. 2.1 KHz 455 KHz center frequency. FA case (as specified in the part number). F455Y21 is the same, but Y case, probably for 75A4. 73, Jerry, K0CQ, Technical advisor to the CRA -- Entire content copyright Dr. Gerald N. Johnson, electrical engineer. Reproduction by permission only.