I have replaced hundreds of Bach stop rod nuts over the years. When someone loses one, the other vibrates than falls off. Result is the third slide dropping on the hard floor, denting and eventually cracking the slide crook.
This nylon stop nut I have is made undersize so that it will screw on and hold itself in place by its own spring tension. Only one is needed. The original ones sell for $4 each (and you need 2 of them). I will sell these replacements for $2.50 each. Expensive for a plastic thing, yes, but I adjust each one by hand, and $2.50 once is certainly better than $4 twice, frequently.