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I think perhaps he's referring to the two files available: LM8UU_igus_style_dia8.3.stl vs LM8UU_dia8.3.stl. I'm a little curious too about this. Comparing the models in Simplify 3D, it appears they're almost the same except the non-Igus ones have channels going perpendicular to the main channels.

I wonder if one is more recommended over the other? Pros and cons?

Screenshot attached