The "X" on the skins is a reminder not to drill the leading holes with the normal #30 or 1/8.
Decided to try updrilling the leading edge dimpled holes with a #36 drill. This is about the smallest size the Simple Dimple Die scarred nail will fit through. Hopefully the resulting enlarged hole will be a little undersized instead of too large. I can then enlarge the holes as necessary for a snug rivet fit and minimal "proud" breaking of the mandrel.