I have a Bacchetta Idler Bolt here. Better image from Bachetta. My issue is the Idler bolt is too long and is causing my Idler to go back and forth, which was fine when I had a cassette, but now I have an IGH, and I don’t need it to slide. The sliding is causing my chain to pop off at the chainring. If I had a smaller Idler bolt, it would not slide.
That looks like a standard front brake nut for older front road rim brake calipers. Any LBS should have some spares laying around to match up visually. There are a few different lengths of these nuts due to the difference in depth of modern carbon, aero bikes, and old steel forks.