When my parents became witnesses almost 50 years into the kingdom rule I believe they would have been part of the 'over a million persons' mentioned in the Watchtower 1966 page 40 who felt they had 'enabled to "see" the sign of the Son of man' by means of the composite sign.
The title of the article is The Sign Of the Son of Man and on page 37 under the subheading The Most Vital Sign it quotes Matt.24:30. On the following page page 38 it says.
What is shown to be taking place here that Jesus could refer to as "the sign of the Son of man"? It is the act of God's Son in recieving authority from his Sovereign Father to rule in an everlasting kingdom. This event brought the establishment or "birth" of his kingly rule. (I think this scripture is now believed to have a future fulfilment)
The article has the next subheading ' How The Sign is Seen' and quotes from the Greek-English Lexicon by Liddle and Scott.
'Concerning the Greek word for "see" used at Matthew 24:30 and Revelation 1:7, their lexicon (Volume 2, page 1245a) lists this as one of the meanings: "Metaphorically,... of mental sight, to discern, perceive," Yes, the Bible speaks not only of seeing with our eyes but also of seeing with our mind and with our heart.' (page 38)
Later on page it mentions the visible sign that follows on 'after and results fom the invisible "sign of he son of man"'
So I think my parents saw this mentally and with their heart.
Having come to be a Witness they then were from my understanding in the spiritual paradise. They had come to the mountain of Jehovah. In the publication Man's Salvation Out of World Distress At Hand! (WTB&TS 1975) on page 183
'The personality of the Messianic King affects the personalities of his subjects in the spiritual paradise in an excellent, upbuilding way. The prophecy of Isaiah points in that direction.'
It then quotes Isaiah 11:6-8 and on page 185 it says
In harmony with that intention, such a transformation as is foretold by the prophecy regarding the animal world has actually taken place in the spiritual paradise of Jehovah's restored worshipers.
Now my parents are at the end of their lives and i think that Gods kingdom was in their hearts.
The Watchtower 2008 1/1 p13 (Watchtower ONLINE LIBRARY Our Readers Ask Is God's Kingdom in your Heart?)
Many today believe that the answer to the above question is yes. For example, TheCatholic Encyclopedia asserts: “The kingdom of God means . . . the ruling of God in our hearts.” The clergy commonly teach this idea. Does the Bible really teach that God’s Kingdom resides in human hearts?
If i understand correctly what the this is saying I would say it was probably true of my parents. There was no literal government that brought 'dramatic changes on the earth' in their lifetime as they had hoped for.