Single ply hinoki - center part of the tree ?

I noted that on my Darker Speed 90 and Kokutaku J-pen, the center part of the rings showing growth of the tree are on the sides of the blade. This means for these blades the manufacturers use only one piece of wood cut vertically (along the height of the tree). This means that if the useable (good) part is for example 3 meters, and the width of for material for J-pen is 15 cm, they can get only about 20 blades (this will cause the high price). Then perhaps the parts next to the center part are used for lower grade blade. I wonder if my conclusion is correct. Any comments ?

Not sure what you meant exactly.  If you could draw some picture it might help.

The sides of the Jpen that resemble the eyes are not the center of the tree growth rings.  If the side of the Jpen were cut dead straight instead of the slightly round curve, you won't see those those concentric oval circles - "the eyes."  Those eyes signify the growth lines along the side of the blade are straight.  The more and identifiable eyes you see would indicate the growth lines in that area (or years to be specific) are straighter and more packed together (or it means this particular Hinoki tree in those years got even precipitation, no drought, no sudden climate change, and suffered no trauma).

My opinion is that they are not near the centers of the tree otherwise if you look at the bottom end of the blade, the lines would appear curved. Instead they are mostly straight indicating they are more likely to be taken from parts a distance away from the center so the lines appear parallel and quite straight.