Link to a thread about 55 that has a lot of information, some lore, and much speculation. One thing that mystifies me and contradicts my findings today is about the base used for the 55 negative. I find the polyester base of TMX is a more exact match than any piece of Panatomic X I could did up. There is a 0.002" thickness difference, but the material in both is PET and not acetate.
Here is the link.