Somewhere in my garage is a recoil plug, like above, it was launched into some never to be found place. So for that pistol, I ordered two spares. On one of my full custom guns, the barrel pad where the link is attached just sheared off, requiring a new fitted Barstow barrel, under warranty. I did not have a spare fitted Barstow barrel for that pistol. Can't plan for every eventuality, so like above, I would just suggest the usual wear items, like springs and maybe a firing pin.
On a related note, long ago in my Army days, I learned to always have a "back-up" for any essential item. But then I began to have a nagging compulsion to push this concept to absurdity. Because once you use your "back-up" item, you no long HAVE a "back-up" item, so I needed to carry a "back-up" for my "back-up", which got to be just a pursuit of infinity.
Carried to extremes, some concepts are just nuts. NV