When you buy nickle/dime parts, get extra. If a commonly universal part like the rivets, get many. (Knuckles do vary some. Some swap, some wont.)
It's easy to foul up the setting of long/tall rivets and one knuckle may work better than the one next to it from th same batch. Test before you set the rivet and mod/file knuckle flash if needed.
As a note, I never set a few of my coupler rivets and they haven't budged let alone fallen out in over 7 year of running, and roll overs etc don't bother me much either. An advantage of running less than mint trains.
