On the authorship of the Pink Letter

Until recently, I always pretty much took for granted that the pink letter was written by Ramsay Bolton and never really questioned it. However I kept reading threads and posts that suggested other potential authors, amongst others, Stannis. I did some research online and found a pretty interesting treatise on the matter which can be read here. It got me to question myself on who the actual author of the letter is and, quite frankly, I really don't have an answer yet. Therefo

Here's the simple reason why Ramsay wrote it: Jon Snow has seen Ramsay's handwriting. He knows what it looks like. Jon gets another letter from the same person. If the handwriting was totally different, he'd have twigged. I mean, Ramsay's handwriting is described by Jon that first time -- the letters are "huge" and "spiky". Pretty distinctive. Stannis and Mance wouldn't know it. Theon might, but he's not exactly in position (nor do we even know he has the skill) to forge a letter. So, i