General Discussion

In the long, miserable Dark Ages, you would carefully dictate your words of wisdom to a scribe, and pay a pretty penny to do so.
After carefully orally editing and writing it out in dense, beautiful and ornate calligraphy, addressing the envelope, preparing the stamp, you would suddenly realize that you had forgotten to tell Aunt Sally that you would attend her daughter's wedding, or the Cubs had just pulled it out in the ninth inning. So your scribe would, if space and penny permit, add this after the X you'd made for a signature. Hence "post" = after and "scriptum" = the written.

Of course nowadays we just edit the text. Or like here, we edit it later and leave behind a mystery edit notation.