Wild guess: If you notice at the bottom of the reply text area there is a "Show your signature" check box.
My guess is that if you don't have a signature yet, it defaults to "no", while if you do have a sig, it defaults to "Yes". Hence all the older posts would have the "do not display" flag set.
Just a guess.
I changed my forum signature about two hours ago. The changes I made are what you see below. I went and looked at some posts I made several days ago and they all have the current (new) signature on them. So it appears that this function DOES work retroactively. So it SHOULD also work for Shdus1.
From what he said, he did not have a sig when he made the earlier posts.
So the "show sig" flag on the post would have been set to false. Just like this post of mine.
Your earlier posts had "show sig" flag set to true. So when you change the sig, the software puts in the new one.
I didn't catch that. So you are probably right.
In any case he has only made a few posts, so simply changing things now and moving on as Gsonline suggested in the best answer.