The rudder trailing edge top rib gets so tight that it's hard to get a bucking bar or squeezer in the aft most position. Because of this, Vans calls out a MK-319-BS pop rivet as an alternate to a driven rivet in this location. When the previous owners built the rudder, for whatever reason, they used the wrong rivets in here. Instead of countersunk, they had just pan head pop rivets. I drilled these out assuming that I was going to have to dimple via a male die on top of a countersink in a 3/8" steel plate slipped in there, but to my surprise, the pan heads was installed over a dimple. Once they were removed, the proper rivet dropped right in with no problems at all. This has been the easiest thing on this project so far. Don't have any idea why they used the wrong rivet. There were plenty of the correct ones in one of the hardware trays that I got with the kit. After discovering this, I looked over the rest of the rudder in detail. Not only for build quality, but also for conformity to print. This was the only area I found where they went "off plan"