Sounds like Russ has had the fun of setting a few of those. We did too, we used a short piece of train track with a divot for the rivet head on our IHC mower sickle. Those nut and bolt replacements would have been nice, but kinda expensive. Grandpa had a tool that he'd screw down over the rivet, had a piece on each side to hold it and pinch it. When we had some that were too long, we'd heat them and then beat them down.
Question about the burr rivets, how long/how much would normally stick up above the leather before setting it with the splash tool? So far I've gotten all mine from http://www.brettunsvillage.com/leather/parts/parts.html
thanks, Mick