What are the differences of these Rögnir versions build wise? There isn't any mention of versions in their website so could it be only different pads?
From what I understand, these are only differences in the ear pads: ear pads with the same thickness, but perforated for the alternative pads and not perforated for the stock ear pads.
The change of ear pads greatly modifies the frequency response of the Rögnir, as shown on these two FR curves.
A more V-shaped frequency response (with more bass) with the ear pads non-perforated in stock, and a flatter, straighter frequency response with the alternative perforated ear pads.
Alternative perforated ear pads for the Rögnir
Version I of the Rögnir (with alternative ear pads, perforated) (more "straight" and "right").
Version II of the Rögnir (with stock ear pads, not perforated) (more bass, more fun and V-shaped).