I thought I saw it posted here but it was on their Facebook page and it says natural milled, maybe somebody else knows more about the size.
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A quick search reveals this:
Milled Leather
Milled Leather is the Surface Structure of the Hide.
The leather is during the production put into the milling machine where it is rotated, comparable to a large clothes dryer just with the absence of air and heat. Through the soft falling action the natural fine lines in the leather are intensified. The industry then speaks from "dinstinctive Milled Leather".
http://www.leatherhouse.com/wissen/lederkunde.php#millkorn