This is an excellent association piece.  It was obtained by the flight engineer on American Airlines Flight 910 from Detroit to New York that day. 

King was in Detroit to attend the funeral of Viola Liuzzo, a white civil rights activist who had been killed by white supremacists in Alabama.  Interestingly, the flight was held on the taxiway after it left the gate so that King could board.  The flight engineer said that in 34 years of flying commercial flights, this was the only flight that he ever saw the airline hold for a late passenger after the aircraft left the gate.

Liuzzo had gone from Detroit, where she lived, to Alabama to participate in the civil rights marches from Selma to Montgomery.  She was horrified by images of the attacks on civil rights marchers on March 7, which became known as "Bloody Sunday."  She went to Alabama and was there on March 25 when the third, and final, march reached Montgomery.  Following a demonstration at the state capitol, she was murdered as she drove demonstrators from Montgomery back to Selma.  The alleged killers were four members of the Ku Klux Klan, one of whom was an FBI informant.  They were arrested within 24 hours, and President Lyndon Johnson announced their arrest on national television.

King was one of several civil rights leaders and other prominent people who attended Liuzzo's funeral at Immaculate Heart of Mary Catholic Church in Detroit on March 30.  Among the others were NAACP Executive Director Roy Wilkins; the national leader of the Congress on Racial Equality, James Farmer; Michigan Lt. Gov. William G. Milliken; Teamsters president Jimmy Hoffa; and United Auto Workers president Walter Ruether.

This signed card, which has never been offered on the autograph market before, comes from the family of the flight engineer.  It is accompanied by the affidavit of the flight engineer, who dated and noted the flight number on the card, and an explanatory letter of provenance.

King has signed with a strong blue ballpoint signature.  The corners of the card are a bit rounded, and there are a slight bend and a small pinhole at the upper right corner of the back, in the stamp area, well away from the signature.  Overall the piece is fine to very fine.