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For both! but he also mentions how you should hold the razor differently for against the grain pass, with the thump on the opposite side of the cutting edge.

Actually yes! And even before that! In 1727 from this thread: http://straightrazorpalace.com/razors/102583-year-1727-razor-illustrated-concave-hollowed.html.

Clearly describe that the thumb was...

Its a wonderful experience, just think of where and what and who this razor belong to, every serious straight razor shaver should have at least one!