That is just plain dumb. They aren't interested in answering your question, and in fact you would have had them send a duplicate to Brother Hounder who would have paid you a visit with two of his cohorts. They want you to just take their word about 607, and there is no way they would have sent you the actual truth about 587.
Here is an idea. Since the Watchtower Society won't answer this question with a straight answer, ask Google. If you put it into your search engine, you will get numerous answers. Try 607 BC, 587 BC, Ancient Jerusalem, and Destruction of Jerusalem in the search engine and see what comes up. If the majority of answers point to the destruction as 587, then there is a good chance that it was 587. And if all the answers that say 587 actually make sense, that greatly ups the odds in your favor. And you will not get a letter forwarded to Brother Hounder, and his cohorts will never know you used Google.