I'd say the .rsr thumbnail format is a one off custom image format used by Metacreations. It has now been abandoned in favour of .png thumbs, which is why pro pack users create items that just have a .png and no .rsr (except for geometry .rsr files which, as I said, are actually a different format despite having the same extension). Did you try the experiment I suggested? I already know the result but I want you to be convinced.

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