By 1959 it seems that the "governing body" and "the faithful and discreet slave" became synonymous. Check out the following 4 Watchtower quotations". (By the way, search your library CD with the two quoted expressions -- "governing body" "discreet slave".)
The following Watchtower deems the two as synonymous.
*** w59 6/15 p. 368 par. 2 Meeting Today’s Needs of the Sheep ***
Sheep need to be fed, not only led. How are they being fed? "The faithful and discreet slave whom his master appointed over his domestics to give them their food at the proper time" certainly is making ample provision. The governing body of Jehovah’s visible organization supplies plenty of good food for all the sheep, including the overseers.
The following Watchtower implies that the "slave" "has a governing body".
*** w60 7/15 p. 438 par. 1 Staying Awake with the "Faithful and Discreet Slave" ***
THE Biblical connections and the abundant array of Scriptural credentials of the "faithful and discreet slave" class have already been examined. This class was found to be the anointed witnesses of Jehovah operating today in all parts of the earth through their now seventy-six-year-old legal servant, the Watch Tower Bible and Tract Society of Pennsylvania. This "slave" class has a governing body rendering counsel and direction as prefigured by Elihu when he gave counsel to ancient Job.
The following Watchtower quotation equates "The Society" with the "discreet slave".
*** w61 3/1 p. 147 par. 13 The Congregation in the Time of the End ***
. This development was completed by 1938 when congregations world-wide expressed their wish to be organized completely under the direction of "The Society," "the faithful and discreet slave" class, who would hereafter appoint all overseers and their assistants in the congregations and who would direct all study and preaching arrangements.
The following deems the two as synonymous.
*** w63 9/15 p. 575 Questions From Readers ***
Even as Jehovah had a channel to serve as an authority or governing body in the days of the apostles and following Pentecost, A.D. 33, so today Jehovah God has a governing body, a "faithful and discreet slave" class, that oversees the work of the Christian witnesses of Jehovah in the New World society.