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This is interesting. It is the retreat of the clecos. With the size drill I used to updrill the holes, the larger clecos will not fit in the enlarged holes.  They will after dimpling, but not until then.  

The first picture shows the updrilling after I had updrilled about four rows.  I started with all the holes on this side having a cleco in each. I remove a whole row of clecos and then drill with a #33 bit.  I continue until all the holes are drilled and there are no clecos on this side.

I then turn the wing over and do the same thing on the other side, as shown in the last picture.  The wing is then finished being drilled and is ready to take apart.