The rivet heads are quite clear, nice circles. Measuring and scaling from the .44-40 cartridge case under the rim, I get a diameter for the rivet head of 0.323", 8mm. The diameter of the splash is equal within reasonable limits. No washers, and this thing has held together for this long.... When you look at it, there isn't that great a force trying to tear the rivet out, just holding a friction fit on a cartridge case.
God bless
Hope you can see the two rivets, or at least the one on the left.