Sorry for the lack of response. I just came across this. I am not the expert on the particular version of software or the specific set-up we are running; however, I would have to say, most forum servers are not set-up to make a back-up archieve of every post in every thread. It would take a great deal of resources from the server(s). I would think that dependent on set-up there could be a timed archieve or a particular event that triggers an archieve. I would find it hard to believe that we archieve back-ups upon edit of every post though.
I can confirm this with one of the admins if you like. Is there a particular reason that you are asking?