The info you are missing is Step 5
Step 5: Machine countersink the #30 holes in the
VS-808PP doubler that attach the doubler to the
VS-803PP Rear Spar for the dimple of an
AN426AD4 rivet. See Figure 2 for locations.
Dimple the corresponding #30 holes in the rear
spar for AN426AD4 rivets.
Since dimples and countersinks only apply to flush rivets, this step will only apply to holes where a symbol for a 426 rivet is located (the flush head fwd side zone at the bottom).
So, since the flush head is fwd, you dimple the spar web to be flush on the front and then machine C.S. the fwd face of the doubler to allow for the dimples. (Refer to Section 5 for details on countersinking for a dimple).
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RV-6A (aka "Junkyard Special ")