A CS4-4 rivet is a Counter Sunk / flush rivet.
Because of that, the hole it goes in must be prepared in some way for a flush rivet (dimple countersunk or machine countersunk).
The hole pointed to by the arrow was specified to be machine countersunk in Section 20iS/U, page 3, Step 8, for a CS4-4 countersunk rivet.
The riveting statement means to say the countersunk hole at each end of the bulkhead (it assumes since you are installing a flush rivet, you will be looking for holes that are either dimple countersunk or machine countersunk. At this point in the assemble process there are only two. The upper hole for attachment of the two F-01223 ribs.
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RV-6A (aka "Junkyard Special ")