The quote seems to be accurate. I found something extra: 1910 Watchtower.
It's from a book by Edmond C. Gruss, Jehovah's Witnesses: Their Claims, Doctrinal Changes and Prophetic Speculation. What Does the Record Show? The book seems to discuss claims made by the Watchtower such as this one, amongst many others.
After 5 minutes . . .
Yeah, I found the whole reference.
If the six volumes of SCRIPTURE STUDIES are practically the Bible topically arranged, with Bible proof-texts given, we might not improperly name the volumes-- the Bible in an arranged form. That is to say, they are not merely comments on the Bible, but they are practically the Bible itself, since there is no desire to build any doctrine or thought on any individual preference or on any individual wisdom, but to present the entire matter on the lines of the Word of God. We therefore think it safe to follow this kind of reading, this kind of instruction, this kind of Bible study
Furthermore, not only do we find that people cannot see the Divine Plan in studying the Bible by itself, but we see, also, that if anyone lays the SCRIPTURE STUDIES aside, even after he has used them, after he has become familiar with them, after he has read them for ten years --if he then lays them aside and ignores them and goes to the Bible alone, though he has understood his Bible for ten years, our experience shows that within two years he goes into darkness. On the other hand, if he had merely read the SCRIPTURE STUDIES with their references, and had not read a page of the Bible, as such, he would be in the light at the end of the two years, because he would have the light of the Scriptures.
Our thought, therefore, is that these SCRIPTURE STUDIES are a great assistance, a very valuable help, in the understanding of God's Word. If these books are to be of any value to us it must be because we see in them loyalty to the Word of God, and as far as our judgment goes, see them to be in full harmony with the Word and not antagonistic to it. Therefore, in reading them the first time, and perhaps the second time, and before we would accept anything as being our own personal faith and conviction, we should say, "I will not take it because these studies say so; I wish to see what the Bible says." And so we would study the Scriptures in the light of these SCRIPTURE STUDIES; we would prove every point, or disprove it, as the case might be. We would be satisfied with nothing less than a thorough investigation of the Bible from this standpoint.
To my understanding, the message Charles gives in the above is contradictory. He says that the Watchtower is the Bible itself because if God had any further message to give to his people, he would do so through another medium—such as the Watchtower, according to Charles's implication. However, he also says that the Scripture Studies are "a very valuable help in the understanding of God's Word." I think he is implying the same thing the Watchtower does today—namely that the Watchtower gives the same message as the Bible, so, even though you are encouraged to read the Bible as well, you should go along with the Watchtower's interpretation.