If a second nut on that bolt is necessary for better performance of the shifters, I've got a public service announcement here.......

I'm still trying to track down a candidate nut that will fit that I can cut or file a slot into, but in the meantime I found a small hex nut at Home Depot that fits on the bolt. I believe the size is a M4-7 or M4.7. It is not as wide across as is the original, but it fits on the bolt and applies pressure on the other nut.

Package also reads: 18-8 Stainless Steel Br-M / 12-M. A package of two cost me 50 cents plus tax. Found them in the sliding drawers.