By the way, if anybody is looking for some of what Russell originally said, several of his "Studies in the Scriptures" are online, hosted by the North Seattle Bible Students at:
http://www.nsbible.org/
Look under the "Library" link. All the original 606 stuff is there. Interestingly enough, he dated the ending point to the time when Cyrus freed the Jews, not when they actually got back, as the WTBS claims. When, in fact, 70 years ended at or before Babylon was overthrown (Jeremiah 25: 11 - nations shall SERVE Babylon and verse 12 -after 70 years are completed, I will punish the king of Babylon).
History says Babylon was overthrown in 539. History also tells us that Nebuchadnezzar's father, Nabopolasar freed his people from the Assyrians in 609 BC and thus took over - his land and surrounding lands. That's your 70 years right there. The 70 years that "these nations" will SERVE Babylon. And at the end of those 70 years, Babylon was overthrown, exactly as the Bible says.
Note also 2 other 70-year periods in the Bible. One in Isaiah applies to Tyre, so discount it. There are also 70-year periods mentioned in Daniel 9 and Zechariah 1 and 7. These are all either saying 70 years still hasn't ended or has just ended. These are during the reign of Darius, who history says reigned from 522 to 486 BC. Zechariah 7 talks about the 4th year of Darius and "these past 70 years" or something like that. His fourth year (assension, 1,2,3,4) is in 518 BC. Back up 70 years, you come to 588BC, when Nebuchadnezzar first laid siege to Jerusalem (defeating it 1.5 years later either in 587 or 586).
Even if you don't buy the 70-year chronolgies in Jeremiah, this one starts in 588 and extends way into the time after the Jews return to Jerusalem. How can this date be explained? What triggered the start of THIS 70 year period if the Jews were safely tucked away in Babylon at the time? Maybe the JWs twist that too so that Darius ruled 20 years earlier or something - haven't checked yet.
Basic dates are:
609 - Nabopolassar frees his people and begins to take over the surrounding nations
605 - Nebuchadnezzar beings ruling
588 - Nebuchadnezzar begins to lay seige to Babylon
587 or 586 - Nebuchadnezzar conquers Jerusalem
539 - Babylon is overthrown by Cyrus
538 or 537, Jews return
518 - another 70 year period ends