B.L.E.

I assembled a kit and in the assembly instructions was a statement that due to the tendency of paper to expand and shrink with humidity, the precut notches in the leading and trailing edges for the ribs may not exactly match the spacing on the plans. So, I think that it may be a result of the original plans shinking or expanding. We used to shrink silkspan and tissue by wetting it in the good old days before Monokote.