As promised. The one on the left in the picture above is a knot cup brush while the one on the right is a standard wire brush. As you can see, the wires in the knot cup version are wound together into bundles. This helps a) keep the wires in the brush, and b) helps to assure that if they do come off the brush, they’re not straight, sharp wires.
The one pictured is from Northern Tool and clocks in at $12. There are plenty of others on the market, too.