As far as I can tell, this was fairly blurred for a long period - and probably still is. Rutherford's book, Riches, (1936) introduced the idea (using the principle of types and antitypes) of an "Elisha" work beginning in 1919 following on from the "Elijah" work of the preceding period.
So when was the "faithful and discreet slave" appointed? That was never clarified - this was my summary in 1996:
"The doctrine of the "faithful and discreet slave" allows room for the view that the Watch Tower movement, as constituted under Russell's leadership, was already Jehovah's organisation, and that its commissioning for the prophetic role of the antitypical Ezekiel did not amount to its first becoming Jehovah's organisation." (Counting the days to Armageddon, p121.)
The trouble was that every item in Russell's end-time calendar had been reassigned a date in a new calendar beginning (instead of ending) in 1914. But Russell's understanding of his movement's place in the divine plan could not similarly be transferred to the new era without throwing away the foundation on which Rutherford had built. So this problem was left unresolved. And it looks like the Wise Old Men are only now getting around to tidying things up. To get away with it they will need Russell to be completely erased from their history.