As mentioned by others, various aspects of this teaching (British Israelism) were not unique to J.H. Allen, or to HWA, but if you specifically compare Allen's book with HWA's you will find some undeniable similarities in specific phrases, etc. between the books. For example, HWA used the term "Race, not Grace" in chapter IV of his book, while Allen used a similar term to title chapter II of his book. HWA also used an analogy mentioning "Californians" in chapter VI of his book... "Jews are Israelites, just as Californians are Americans. But MOST Israelites are not Jews, just as most Americans are not Californians." Allen's book states..." But the great bulk of Israelites are not the Jews, just as the great bulk of Americans are not Californians, and yet all Californians are Americans;..." chapter V.
All this to say that many of the "truths" attributed to HWA which the COGs claim were "restored" by him were actually taught previously by others, fifteen to fifty years or more before he ever came on the scene in any public way. Not all of these men were necessarily Sabbath keepers, but some were, and some had ties to the Millerite movement in the 1800s. Their books, articles, etc. were circulated and would have probably been available in libraries with religious sections. For example, Russell's books entitled Studies in the Scriptures printed around twenty million copies in various languages and were distributed all over the world, according to Wikipedia. You can still find them in print available on Amazon.
If we in the COGs claim to uphold "truth" then we also have to be willing to uphold "truth" even when we find it doesn't fit the narrative we have been told, and many among us still believe. I am not speaking here of the legitimacy or not of some of these specific doctrines, which people who come to this blog will hold different opinions on, and would involve a much longer conversation. But, we have been taught a certain narrative of our own church history and its founder that isn't completely accurate when one delves into the actual facts of the matter, and we become hypocrites if we continue to promote a false narrative to those who come after us, when the facts are there for anyone to see and research for themselves.
For those interested in doing some follow up on Charles Taze Russell, you can find some of his writings here... http://onlinebooks.library.upenn.edu/webbin/book/lookupname?key=Russell%2C%20C%2E%20T%2E%20%28Charles%20Taze%29%2C%201852%2D1916
A direct link to the Journal article as mentioned in the above comment is here...http://www.thejournal.org/issues/issue202/jx013118.pdf
In the above article, a man by the name of George Storrs is also mentioned as being a mentor of Russell, and a volume of his Six Sermons can be found here... http://watchtowerdocuments.org/documents/1856_Six_Sermons_Storrs.pdf
As others have mentioned, G.G. Rupert was a proponent of keeping the Biblical holy days. Some of his writings can be found here... http://www.friendsofsabbath.org/ABC/GG%20Rupert/ggrupert.htm
An online version of Judah's Scepter and Joseph's Birthright can be found here...https://originofnations.org/books,%20papers/JUDAH'S%20SCEPTRE%20AND%20JOSEPH'S%20BIRTHRIGHT%20(J%20H%20Allen).pdf
Or here...http://www.christianchurchesofgod.com/books/jhallen/start.html
by Concerned Sister