At the time of the appointment of the faithful slave in 1919 it was understood to include all anointed Christians, and at that time all Christians were anointed. So it's a bit of nonsense to say that the faithful slave was appointed at Watchtower Headquarters. Or to say it included Watchtower directors as it included everyone. Or to speak of the appointment being passed on to subsequent Watchtower directors.
It was only in 2012 that there was a change in understanding that the faithful slave only referred to the Governing Body collectively (not individually).