Hi Fernando,
the amount of decoration on the sheath...opposite to the 'window' side...suggests that this would be the facing side. So the question is......why is there a window
It would make sense that you could make a weapon look larger (and therefor have more status) by putting a knife/sword in a scabbard that was too big. If this was a common practice perhaps the 'window' was a way of showing that the scabbard was 'full of blade'......the only thing is ...the window would have to be on the facing side...so that it could be seen.
If I am right and the blade window faces the body, then, perhaps, there could be some cultural/religious reason.....I can see no practical reason to leave part of the blade exposed to the 'elements' (humidity etc)