Walter died on March 22, 1961. Here’s his obituary from the Washington Post. The archive also has copies of obituaries from several trade and research publications.
Here’s a more personal description of the events leading up to Walter’s death, from his youngest son (and my father), Tom:
My Dad died of respiratory cancer on March 22, 1961 at Anne Arundel General Hospital in Annapolis, Maryland. He was 66 years old. The years of cigar smoking were the cause.
Mother (Ina) and I were there. He had gone into a coma a day or so earlier and I came back from college. I remember that I had not taken time to change out of my Air Force ROTC uniform.
He had first shown signs of not being well about 1957, when they took me with them to Europe for a month. He was delivering a paper at a conference in Hamburg and meeting with Dr. Goodwin-Bailey in England, who was working in the same field as my Dad. They were old friends.