It is interesting to note, that most of the other translations (non NWT) do use the word "for" in english. I looked it up in the NASB, KJV, NKJV, Websters, RSV, Youngs, Darby's, ASV, Hebrews names version. Of these versions it is only the KJV, NKJV, and Websters, that uses the word "at", Youngs uses "of", all of the others use the word "for".
Now according to my studies I have found that the written date that Jerusalem fell to total destruction was in 597 according to the Babylonian Chronicals. The Babylonian Chronicals are a large set of tables that were actually a copy of the set of event that took palce during the times of the Babylonian Captivity. The translation of these tablets was started in 1887. In 1956 there were a set of smaller tablets that when translated, it was discovered that the Destruction of Jerusalem came in 597bc,in the 19th year of Nebuchadnezzar. The tablets also established many other dates including the time periods that the Babylonian kings ruled. According to the WTS and the chronology they use, Jerusalem fell in 607bc yet when we look at the tablets Nebuchadnezzar was not even king during that time, he did not take the throne until 604bc. If you would like to check out a research link dealing with these tablets, go here
> http://www.seminary.georgefox.edu/courses/bst550/reports/DStahlnecker/Chronicles.html
I'm wondering with the falicy that has happened in the continual trying to establish dates, that the WTS is now making an effort to erase the history as they have so often in the past, by rewriting their literature, including the NWT, to bring it in line with what they are now making their doctrines.
Maybe this is not what you were looking for but I hope it helps.
seedy