Stefan, You're going to drive yourself nuts trying to make sense of this brand. Yor fingerboard is most likely "ebonized" rosewood, a fancy way of saying "rosewood stained black". You will likely be able to see the rosewood grain pattern if you study it closely. This technique is widely used in guitar manufacturing Looked at your photos. Nice guitar! Looks as though it could have been made at the same location a my DR 7. I see a lot of similarities to the Tokai(Cat's Eyes)guitars that makes me think some Sigmas were made there. It's very likely there were a few Japanese plants under contract to make this brand that explains the difference in finishes/labels etc. I was interested to see your label quote"Imported and inspected by the CF Martin Organisation" in an above post. Organisation with an s instead of the American OrganiZation, Wonder what that signifies? Probably just a British influence on the Japanese.

Cheers Neil