By the time Russell met him, Barbour had left Adventism for Mark Allen's Church of the Blessed Hope, an Age to Come body. Barbour says his Church of the Strangers was affiliated with Allen's Church. Russell says as much too without naming Allen's movement. Russell wrote:
The Answer ... explained how Mr. Barbour and Mr. J. H. Paton, of Michigan, a co-worker with him, had been [Italics are mine.] regular Second Adventists ... and that when the date 1874 had passed without the world being burned, and without their seeing Christ in the flesh, they were for a time dumbfounded. They had examined the time-prophecies that had seemingly passed unfulfilled, and had been unable to find any flaw, and had begun to wonder whether the time was right and their expectations wrong, – whether the views of restitution and blessing to the world, which others were teaching, might not be the things to look for.
Notice that they HAD BEEN Adventists. They were such no longer when Russell met them, but had shifted to the teachings of "others." Restitution doctrine is Age to Come doctrine.
George Storrs left the Millerite movement [Adventism] in 1844 among much controversy and recrimination. When Russell met him (1874) he was teaching, not Adventist world burning, but Restitution doctrine. He wrote in Bible Examiner that he did not and had not for some time read any Adventist publication. He did, however, write for Age to Come journals including the Restitution. Adventists sniped at him, defamed him, and printed lies. He addressed this in issues of Bible Examiner.
B. W. Schulz, a Fellow of a scholarly group focused on Witnesses, that includes the most prominent writers of today, and my mother, Rachael de Vienne, are, I would say "the most senior" writers on Witness history. Their work is cited by the most recent authors. These include George Chryssides in Jehovah's Witnesses: Continuity and Change and Zoe Knox Jehovah's Witnesses and the Secular World. Penton's third edition also cites their work and calls them "first rate historians." Mom and Dr. Schulz's books are narrative changing, and their conclusions are backed up with citations and quotations from original sources. Bruce contributed fact checking and editorial comments to Chryssides forth-coming book, due out in September, and he is acknowledged in the preface.
That someone who wrote some years past does not include current research proves nothing but that he was unaware of developing research. That you can't find references to Age to Come only means you looked in the wrong places. Start here: https://www.google.com/search?q=%22age+to+come%22&tbm=bks&source=lnt&tbs=cdr:1,cd_min:1800,cd_max:1899&sa=X&ved=0ahUKEwjVx9KmrNPvAhXbLc0KHVRXCtkQpwUIJQ&biw=1366&bih=625&dpr=1
The Age to come movement, sometimes called Literalist, was diverse with many sub doctrines that conflicted with others within their movement. Russell was most influenced by those associated with The Restitution. By the time Russell met him, Stetson was teaching age to come and had been since 1865. He says so in a Restitution article. While some of his former Adventist associates still admired him, Stetson was dropped from the Advent Christian speakers list because of his change in doctrine. His articles were no longer accepted. And he was writing for The Restitution and for the British journal The Rainbow.