[All parts of this post may be found here.]
From Roberts’ death (1933) up to 1960, First Council members were called from the ranks of the seventies quorums and the elders. Still no high priests allowed. The First Council visited missions and stakes, but could not perform much administrative work there. Then in 1960, David O. McKay decided that the current members of the First Council could be ordained high priests and yet maintain their membership in that body. The news rocked the LDS world a bit (just a bit–the reason: HC 2:476. Check note 3 in part 4 for why that angst was probably unfounded). This move opened the way for the First Council to perform administrative work in the stakes on an as needed basis. They went when the 12 or their assistants could not, or they (the 12) formed teams with the First Council, training them in place. The members of the First Council had never come out of the ranks of the local church leaders like bishops or stake presidents and consequently were seen as somewhat eclectic in their approach. Now things were different. The flow from below would change.
A few years later, the First Council was passed the baton of sealing. This step got little press, but it was important and a real extension of the previous century. The First Council were effectively becoming “Assistants to the Twelve.” The growth of the Church would be the final factor in the change (end) of the First Council.
After 1901, the rank and file seventies gradually became less involved in foreign missionary service. More and more they became “home missionaries.” Staffing home-based efforts, serving in ward and stake teaching calls, but still having quorum alliances unrelated (directly) to ecclesiastical units. They were becoming middle-aged men for the most part who were stuck in a puzzling branch of the kingdom. It would take 85 years to mend things. The number of seventies quorums continued to increase, but their role was essentially confined to local missionary activities.
In 1967 a new class of church officers was born, the Regional (or Mission) Representative of the Twelve. They would be high priests and could act in a regional setting (they were not general authorities) in regulating stakes and missions (at first they were conceived as line authorities [see part 6], but that was revoked after a number of problems). This, like the Assistants to the Twelve was seen by the First Council as another move ignoring the text of D&C 107. Weren’t the members of the First Council “the true assistants to the Twelve?” The Regional Reps signaled a new development for the Seventy, but it would be decades until that flowered.
The colorful dynamism of the First Council that was maybe a corollary to their unique leadership path (names like Bruce R. McConkie, Paul H. Dunn, A. Theodore Tuttle come to mind) was about to give way to a new framework that pushed them into a process already in evidence in the 20th century: apostles now came out of the ranks of local leadership. That’s for next time.
 First Council members were ordained (mostly) in June 1961. McKay made it a choice for First Council members. Senior President Levi Edgar Young declined for example. Handbooks issued following this move list First Council members as authorized to call and ordain bishops, set apart high priest quorum presidents, stake presidents and stake patriarchs. Not everyone was happy about the move. For example see Prince and Wright, David O. McKay and the Rise of Modern Mormonism. p. 150. (Observe Harold B. Lee’s remark on HC 2:476 there. While he supported McKay, he didn’t care for the change himself.) Frankly I see this as a way to systematize the whole shootin’ match. Everybody’s a high priest (except stinky elders) but with special ordinations for particular duties. I doubt they’ll take me up on this. (Brigham’s influence is too strong here. Not even JFS could break it. But I think the mythos could be effectively reinterpreted.) Also see Edward P. Kimball’s wonderful bio of Spencer W. Kimball, Lengthen Your Stride. chap. 25, p. 5 ms. (If you don’t own this I hereby revoke your membership in the BCC club.)
 A summary is Ouellette, “Seventies Quorums,” Sunstone Magazine (Jan. 1987) p. 35ff. On this dissatisfaction, S. Dilworth Young, private communication to me and see Ed Kimball above. Also, see S. Dilworth Young, “The Seventies: A Historical Perspective.” Ensign (July 1976). Also, Earl C. Tingey, Ensign Sept. 2009.