Hi Chris,
In JW theology, the heavenly calling commenced and the anointing began at Pentecost of 33 CE:
"Thus the ancient Festival of Booths pointed to the joyful ingathering of redeemed sinners into Jehovah's great spiritual temple. The antitype of this festival started at Pentecost in 33 C.E. with the commencement of the joyful ingathering of spiritual Israelites into the Christian congregation. (Acts 2:41, 46, 47) These anointed ones appreciated that they were "temporary residents" in Satan’s world because their real "citizenship exists in the heavens. (The Watchtower July 1, 1996 p. 24)
"While the Jews were celebrating the typical festival of weeks, or Pentecost, at Herod's temple in Jerusalem, about 120 disciples of the spiritual High Priest, Jesus Christ, were met together in an upper room in that city. Then, before the third hour of the day (9 a.m.), the Lord Jesus Christ poured down holy spirit from the Most Holy of Jehovah's spiritual temple. This resulted in their being spiritually begotten by their heavenly Father, Jehovah, and their being anointed with holy spirit by means of their High Priest, Jesus Christ. (The Watchtower February 15th 1980 p. 11)
JW theology teaches that there has been only this one calling from Pentecost up until the time of the end:
"At Pentecost of 33 C.E. the first of them were anointed with holy spirit. (Acts 2:1-4; 2 Cor. 1:21, 22) God's provision for salvation leading to immortal heavenly life was made known. Peter used "the keys of the kingdom of the heavens" to open up this knowledge: first to the Jews, next to the Samaritans, and then to people of the Gentile nations. (Matt. 16:19) Special attention was being given to making up the government that would rule mankind for 1,000 years, and nearly all the inspired letters in the Christian Greek Scriptures are primarily directed to this group of Kingdom heirs—"the holy ones," "partakers of the heavenly calling."
Their being called to heavenly life was not because they were somehow better than all the servants of God who had died before Pentecost of 33 C.E. (Matt. 11:11) Rather, Jehovah now had begun to select those who would be associate rulers with Jesus Christ. For some 19 centuries after this there was only one calling, the heavenly one. It was an undeserved kindness that God bestowed on a limited number in furtherance of his own wise and loving purposes." (United In Worship p. 111)
The doctrine of the "Faithful and Discreet Slave" in its present form cannot exist without the two ideas above, because it holds that the "Slave class" has fed itself in an unbroken chain since its inception
"The situation may be compared to that of the faithful and discreet slave that gives food to the domestics at the proper time. (Matthew 24:45) The slave as a body is responsible for supplying the food, but the domestics, the individual members of that body, are sustained by partaking of that spiritual provision. They are the same group but described in different terms—collectively and individually." (Revelation Climax p. 201)
"Apparently one generation of the "Slave" class fed the next succeeding generatio thereof" (God's Kingdom of a Thousand Years p. 344)
"Down through the years the slavelike congregation has been feeding its true members faithfully and discreetly. From Pentecost, A.D. 33, up to this very present hour this has been lovingly and carefully performed." (The Watchtower July 15, 1960 p. 435)