On a matter of form

I wrote an e-mail yesterday where I used a PS (post scriptum) and then a PPS (post post scriptum). Then last night while wandering across the Blogiverse, I found where someone had used a PS and then a PSS. I started thinking about it. I was taught that PS/PPS was the correct form, but when I mulled over it for a while (boring evening), either one makes sense.

If post post scriptum is translated as after, after writing, then post scriptum scriptum would be after writing, writing. It can make sense either way.