I am just going to throw this in here, we do have a wiki, and though it is a bit empty right now, there are some sirens with production sates, so you can go look them up so you don't have to ask about dates and have "I guess" or "I think" answers to that, and a few other bits of info.
Also, even though it has been said already, I'll say it again to keep OT. It has been confirmed from Federal that the 500 is the direct replacement for the Thunderbolt, while not being made right after, it is meant to be its replacement.