Understanding the Use of “of” in a Sentence: A German Question

Hello everyone!

I have a question about learning German. Could you help me parse this sentence?

example sentence:
“They photographed, measured and numbered rubble stones. Each one individually. And then they had to know where each stone used to be in the whole. Because every stone has to go back to its exact place.”

Ask:
1. What does the word “of” mean here? Is the preposition “of” governed by the verb “to know”?
2. Is this sentence “where its place in the whole used to be” an object sentence? If so, has a pronominal adverb “of it” been left out in the sentence “And then they had to know about everyone”? e.g. “And then they had to know from everyone where their place in the whole used to be.”
3. Can I just say “And then they had to know about where his place used to be in the whole”? (of each→thereof）

Thanks in advance!