PD- On page 162 of the Four Presidents Book, under the heading What Was The Mystery Doctrine?, says: Penton provides these brief explanations: The Mystery, as understood by Russell, meant that not only had Jesus, the man, offered himself as a ransom sacrifice for mankind, but the body of Christ, the 144,000 members of his church, also participated in the ransoming and atoning work.
[Russell] felt that the mediator for the New Covenant was "the Christ, head and body." That expression meant, of course, that the 144,000 actually participated in Christ's sacrifice for the world and were joined to him as his "bride" or his "body."
I have just responded because I found you have asked again under another, more recent thread, too.
Don't know if this helps. The Four Presidents Book explains more.