[David O. McKay]
One reason why more urgent steps have not been taken to replenish quorums is the fact that we are now just passing through a transition period in regard to the ordaining of Seventies. As you no doubt understand, all worthy Priests, after having completed a two-year course, are now ordained Elders with a view to receiving two years training as such. At the conclusion of this training worthy Elders who are prepared will be called upon missions, and will be ordained Seventies. When this plan becomes operative you will readily see that Seventies quorums will be replenished by the young returning missionaries.
In the meantime, however, if you need Seventies to carry on your local missionary work, Elders who have already fulfilled missions, and who are fitted for this home service, may be properly recommended and ordained to the office of Seventy.
[David O. McKay, Letter to Peter A. Hard, 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]