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The first picture is mid riveting of the drive horn assembly.  The bend in the drive horn has to match the slight angle cut into the bottom of the rudder skin. That cut is in process in the second picture.  That 1/8" hole will form the radius between two cuts yet to be made along the short and long blue lines coming from it.

I test fitted the drive horn and my Dad is holding the hinge halves that we drilled.