Thanks, Jim is great (on a good day) but I have a set of them and used them as a pattern to make some bolt in versions. You start with a Stainless carriage bolt and turn off the square section under the head and then turn the head to match the rivet head. When it is all bolted up no one will see the nut and there is less (never say no when working on a freshly painted car) chance of damage to the chrome.
Cheers
Arno