Russell had a complex theological system which he formed from various sources, but he did not always credit his source.
There are two potential sources for his view of the 144,000:
One is Henry Dunn. Russell was familiar with Dunn - he mentions him in the supplement to the first issue of the Watch Tower as a forerunner. Dunn's restitution views were spread by George Storrs and Jacob Blain, both active among the Adventists. Dunn tried to reconcile seemingly contradictory scriptures: that there are many that are saved, and that there are few that are saved. This is how Dunn answered this question : the many saved are the World in a future probation, and the few saved were the Church selected now to in the future reign over and serve the World. So Dunn taught two salvations. Quoting Dunn : the election of the few are for the benefit of the many . (Russell took this view over and made it a central part of his doctrinal system).
Dunn also wrote of a "secondary spiritual class", who failed to make their calling and election sure, and yet were saved eternally. Dunn viewed election as meaning not eternal salvation but election to the highest rank in the kingdom - kings and priests. Dunn noted that there were parables of reward, and distinctions of rank. Russell named the elite class "The Little Flock" and this secondary spiritual class "The Great Company".
Another source of Russell's view was the minority opinion of the "partial rapture" theory - that the saints are raptured in groups, the first group is more faithful, and the next group less faithful. This "partial rapture" theory was held by Joseph Seiss, the most prominent American Lutheran minister of the time, who was editor of a prophetic journal called The Prophetic Times. The writers in this journal often speculated on how many raptures would take place, and what order. G. N. H. Peters, also a Lutheran minister, and contributor to the Prophetic Times, and a friend of Russell, also held to the partial rapture theory, and wrote a 3 volume work "The Theocratic Kingdom". The view that there would be two raptures is based on Rev. 7: the 144,000, and the Great Multitude, or "Great Company". Russell called the 144,000 the Little Flock, and considered it a literal number. Also, the parable of the wise virgins (Little Flock), and foolish virgins (great company).
He considered both to be spiritual classes. However, he also believed there would be an earthly class after the Great Company, made up of those who consecrated after the door to the high calling, spiritual calling, was closed, similar to the earthly class of the Ancient Worthies of the Old Testament. However, Rutherford considered the "Great Crowd" to be an earthly class. I wonder if Rutherford merged the "Great Crowd" class and the "Modern Worthies" class or confused them.
Getting back to Russell's view of the Little Flock: he expected the rapture of the Little Flock in 1881. This did not occur. He then changed his view of the rapture, and taught that all the Little Flock would die. He next expected the change ( how this was to happen was not clear) to occur by 1914. This also did not occur on schedule. He then interpreted the story of Elijah and Elisha as a picture of the separation of the Little Flock and Great Company. Elijah was the Little Flock and Elisha the Great Company. The taking of Elijah to heaven was interpreted as a type of the rapture ( as in Russell's book with Barbour called the Three Worlds) , but Russell had modified the theory of rapture. After Russell's death, the Bible Students splintered into various groups. Instead of the Great Company being revealed by the supernatural event of the rapture, some thought it to be revealed by which Bible student group you were in. (Timothy White's book has a good explanation of the Elijah and Elisha views.) Ones were condemned as being of the Elijah class, Great Company, and lower and lesser than others. (This was an unfortunate consequence of Russell's confusion on this matter).
My own opinion about this:
Dunn's views were stated tentatively, as inferences, and explained in detail his reasons and basis for his views. Russell took them over and stated them dogmatically, and did not always give his reasons, but assumed it to be true. The theory of the two salvations of Dunn and Russell has merit and is likely the most attractive aspect of Russell's system. It is possible that there are indeed distinctions of rank in the kingdom of God, known by and rewarded by God. However, this is a different matter than a division among Christians in this life. But Russell believed that the endtimes had already started,and that division was imminent. This division of Christians into higher and lower classes has not borne good fruit. It has led away from charity and unity, toward spiritual pride, legalism, judgmentalism, and spiritual abuse, and confusion over one's class. It would be better to view all Christians as equals, and leave the judgment up to God.
Steve