Lowry Nelson describes his reaction to the First Presidency letter on the priesthood restriction and expresses his opinion that J. Reuben Clark authored the response.
Lowry Nelson, In the Direction of His Dreams: Memoirs (New York City, NY: Philosophical Library, 1985), pp. 340–341
As much as I was "stunned" at Herber [Heber] Meeks' question regarding my knowledge of the existence of people of "pure white blood," this letter from the First Presidency was shocking. I knew these men quite well, especially Presidents Smith and McKay. I knew all of the McKay family; the youngest son, Morgan, had been a member of my fraternity in Logan. I referred in an earlier chapter to the assistance of Dr. and Mrs. George R. Hill during my illness. Mrs. Hill was a sister of David O. McKay. I adored the whole family. I had associated with President Smith in Boy Scout work. When he came to Minneapolis (in 1943?) he made a special point of having a talk with me. President Clark I knew only casually. He was a member of the First Presidency when I was called "on the carpet" earlier, as already described.
There is no doubt in my mind that he drafted this letter to me. A lawyer by profession, he had spent most of his career in political positions in Washington: Undersecretary of State in the Harding-Coolidge era and Ambassador to Mexico under Hoover. It was while he was ambassador that he was named a Counselor to President Grant. However, he continued to spend most of his time in New York with the Foreign Bondholders Association until the 1930s.
But the draftsman is only indidental. This was the law and gospel on the subject.