Originally, (And by "Originally", I mean the doctrine as it was introduced in 1935) the terms "Great Crowd" and "Other Sheep" were synonomous. The "Other Sheep" were declared to be an "Earthly Class" because the "Great Crowd" was an "Earthly Class."
Actually, it's rather the other way around. Russell believed that the Great Crowd was a heavenly class (though less faithful than the “Bride Class” [144 000]). Rutherford taught the same until 1935, when he identified the Great Crowd as being made up of Other Sheep. The Other Sheep had long been believed an earthly class.
Russell’s teaching was that the Other Sheep [John 10] and Sheep [Matt.25] both were an (i.e. ONE) earthly class to be separated in the Millennium. (See the article “Other Sheep Not of This Fold” [WT Reprints page 4784]). In the 15. October 1923 Watch Tower, Rutherford retained the belief that these Other-Sheep/Sheep were an earthly class, but changed the time of the separation of the Sheep and Goats. The Other Sheep [John 10] and Sheep [Matt. 25] parted ways (became two groups) after the change of the WTS' interpretation of the parable of the Separation of the Sheep and Goats. Regarding the Sheep and Goats, the 1997 QfR concludes with the words: "As The Watchtower showed, Jesus will soon sit on his throne and pronounce sentence, people being judicially separated in a final sense to life or to cutting-off." In contrast, the Society still teaches that the Other Sheep are not declared righteous for eternal life in a final sense until after the Second Test following the Millennium. (Rev. 20)
My question remains: Who are these Sheep of Matthew chapter 25? The Society can no longer identify them in a manner consistent with their current teachings.