John Aquila Brown wrote about the 2,520 years prophecy before Charles Russell was born. His book was called "The Even Tide". But his span of dates was 604 BCE - 1917 CE. I can't remember what his 604 date was all about, and if it had anything to do with Jerusalem or some other event.
However, I do remember that Russell accepted the date of 1874 as the invisible return of christ. And there would be a 40 year harvest period, thus culminating in 1914 with Armageddon.
Sooooo, perhaps he just counted backwards from 1914 and reached 607 BCE.
Disclaimer: I do not have all the facts about this. This is merely a possibility. Please do not take this as a fact, because it is just some bits and pieces that I remember from reseraching Russell and 607 BCE a while back.
-mentallyfree31-