The pyramid was erected after his death by J.A. Bohnet a Bible Student. The monument was built in the middle of the WTS plot in the cemetery. It was to be a marker of sorts for the Bible House family (those who worked at the headquarters in Pa). The pyramid contains a cross, crown and wreath symbol, scripture quotes, an open book which has names of Bible Students who died and were buried in the cemetery.
Russell and the early Bible Students thought that God had directed the building of the Great pyramid by Hebrews as a secondary way to confirm the Bible's chronology.
This was not a belief that the Bible Students made up, but was commonly taught in many mainstream Churches which they had just come out of. It took them time to discern what was true and what was myth. In the 1800s some religious scholars felt that the Great Pyramid was built by the Hebrews under God's direction. They believed that biblical texts such as Isa 19:19-20 foretold that the Pyramid contained clues to the interpretation of Biblical prophecies that would be understood in the end times.
But Witnesses corrected their wrong understandings while other religions continue to believe obvious myths associated with the cross and holidays. And there are still theologians and accepted members of mainstream churches who still teach Pyramidology, numerology, and mix "New Age" metaphysics with no repercussions from their respective organizations.
Jehovah's Witnesses have never hidden the fact that they had incorrect beliefs due to ignorance, but we regularly study our history. But, the Bible shows that God allows his servants to gradually understand (Pro. 4:18; Jn 16:12) just as the prophets and Apostles had to gradually learn and change their understandings (Dan. 12:8, 9; 1 Pet. 1:10-12; Acts 1:6, 7; 1 Cor. 13:9-12).
So, concern with past beliefs which were incorrect is a waste of time and distracts from the real issue. That is, a FAILURE to correct false doctrine or actions when presented with solid evidence would be proof that a religion or an individual is not God's organization or a Christian.
It is hypocritical and it's illogical to address past beliefs which have been changed in order to cast doubt on current teachings of any religion. What someone taught years ago has no bearing on the accuracy of what is taught now.
Rejecting current beliefs based on past misunderstanding is just plain unintelligent.
(This is an answer provided by Bar_Anerges to a question at Yahoo Answers.)
The Watchtower and the Great Pyramid, then and now (Pastor Russell)
Charles T. Russell and "Pyramid Chronology" (Defend Jehovah's Witnesses)