THis is the original GREEK text : "Τις αρα εστιν ο πιστος και φρονιμος δουλος ...."
'O (=is the definitive article) πιστος (=faithful ) δουλος (=slave)
Just because there is an "o" in front of the faithful and discreet slave that doesn't mean that this is a specific group or singular person at a specific time .Greek language commonly uses "o" for male adjuctives or nouns .For example :
O μαθητης πρεπει να ακουει τον δασκαλο (=The student should listen to the teacher) By using "o" in front of student that doesn't mean that the student is a specific person or group of people at a specific point of time , it shows that ,if you claim to be a student you then have to listen to your teacher.
Matthew 24:45-47 is a parable not a prophecy .99% off all scholars and Christian Churchs regarded as a parable .The slave can be any Christian that demonstates the attributes presented in the verse i.e. faithful and discreet .
The current GB of Jehovah's Witnesses lack any interpratational integrity when it comes to the the Bible ,use your own research and understanding.