Some editions of the SITS have a winged sun on the cover. Did Russell ever publish an explanation for it?
That makes me think of Malachi 4:2, where Jesus is compared to a sun with healing in his wings.

For over twenty-years a lamp emblem appeared on the covers of the "Millennial Dawn" (later changed to "Studies in the Scriptures") series. As of 1911 the lamp was replaced by a winged "sun-disk". This seems to be the favorite point of critics. In harping on the use of this symbol and claiming it to be of the occult they believe they have solid proof Pastor Russell had occult connections, and therefore invalidate all of his life and study in one fell swoop. But it isn't as simple as that. What is the truth about this symbol? Why was it used? What does it mean? Though Egyptians did incorporate this symbol into their culture, it actually pre-dates them by several centuries, putting its creation and use very close to the time after the Great Flood. In fact, archaeologists recognize the winged sun-disk originated from two different types of earthly observations. Firstly, during a full solar eclipse when the sun is totally covered by the moon, there appear for a split second "streamers" or "wings" on either side of the sun. From this primary observation the ancients used a winged sun-disk to represent the domination of the light over darkness. In time it came to also represent that which was dead returning to life. Secondly, at the dawn of the day, when the sun first emerges on the horizon, there is a split second when it appears to have "streamers" or "wings" on either side. This is the basis for the messianic prophecy of Malachi 4:2 (Malachi 3:20 in the Hebrew Nevi'im) "The SUN of righteousness shall ARISE with healing in his WINGS." A sun with wings!

So why did this symbol appear on the covers of "Studies in the Scriptures"? It was a common symbol, appearing as a decoration on many things in the early-mid 20th century. (even automobiles!) But, that's not why Pastor Russell chose it. He used it for the same reason he used the lamp symbol: The light of Truth was shinning bright! By 1908 Pastor Russell's writings were amongst the most distributed on earth, and it was now clear that the errors of the DARK Ages were being overcome by the LIGHT of truth. Additionally, Pastor Russell taught that Christ had returned, and therefore the Malachi prophecy was already being fulfilled: the SUN of righteousness was already RISING. The dawn of the Millennial day had begun. The Kingdom of God was emerging in its incipient stages.

On a side note this symbol has been traced back to the early years of Israel.

Thank you for that fine bit of information, RR.