RR Auction of Amherst, N.H., released excerpts from a letter Porter sent with the ring, The Dallas Morning News reported. The auction house estimates the gold wedding band will sell for $30,000 to $50,000 in October.
Bobby Livingston, the executive vice president of RR, said the entire five-page handwritten letter dated May 5 will not become public.
"This is the only item of Lee's that has been returned to me, and it took almost 50 years," Porter said. "I'm remarried for many years now, raised my children and have been blessed with grandchildren."
In her conclusion, she spoke of breaking with the past: "At this time of my life, I don't wish to have Lee's ring in my possession because symbolically I want to let go of my past that is connecting with Nov. 22, 1963."
Oswald purchased the ring in Minsk, Belarus, in what was then the Soviet Union shortly before his marriage to Marina Prusakova. She accompanied him back to the United States, and they were living in the Dallas area when Oswald killed President John F. Kennedy and was gunned down himself two days later.
He left the ring in a cup beside his wife's bed before the assassination. Ten days later, it was handed over to the U.S. Secret Service.
In 2004, the ring was discovered in the files of a law firm that had represented Marina Porter and eventually returned to her.
Porter, 72, married Kenneth Jess Porter in 1965. She lives in a Dallas suburb, Rockwall.