This is the postcard shown on Legend Hunter: Jack the Ripper Revealed. It is said there that the handwriting matches that of a known Tumblety letter. Thoughts?
Re the program, the episodes start airing on Travel Channel thisnth, but I've seen several via On Demand. I recommend them. The search for the Irish Crown Jewels was especially interesting.
I just watched this episode today. The letter did seem to match Tumblety's handwriting. Ultimately though, even if that could be determined with 100% metaphysical certainty, it would only show that Tumblety wrote the letter not that he was in fact Jack the Ripper.
I find the host of the show to be kind of a lightweight and rather annoying. He really let on as though he had solved the case while conveniently omitting how Tumblety does not match up with witness descriptions and how homosexual men usually kill other men not women.