P-40LUVR,
ORIGINAL: Let my clarify,one fin,you add ribs to other side to make it whole but only one leading edge stick.
If you glue that one leading edge stick on the plan at first, it will only be on one side of the fin. 2 false leading edges,one leading edge.
I may not understand correctly (enlarged your plan image trying to read the lettering), but it appears that you first form an assembly of the leading edge and the trailing edge, with 23mm 'braces' (shown below FR2 & FR3) glued between them. Onto the 'up' side of this assembly you glue one 'false' leading edge, the fin ribs (FR1, FR2, FR3, FR4) and a piece onto the trailing edge. I am unable to identify the trailing edge piece, but it appears to be a 'doubler', adding to the thickness of the trailing edge. I believe when the glue is dried you then remove the assembly and add a false leading edge, the fin ribs and a trailing edge 'doubler' to the opposite side.
Please think carefully before applying what I've said as I might be wrong!