[Heber J. Grant]
In your letter of February 4 you ask 'When Elder David O. McKay was chosen to fill the vacancy in the Presidency did he, or did he not lose his right to seniority in the Council of the Twelve?'
The answer is no. When a member of the Council of the Twelve is called into the quorum of the First Presidency he retains his place of seniority in the Council of the Twelve.
[Heber J. Grant and David O. McKay, Letter to Jesse W. Richins, as quoted in Minutes of the Apostles of The Church of Jesus Christ of Latter-day Saints, 1910-1951, Privately Published, Salt Lake City, Utah 2010]