It would help if that part name existed. Maybe a parts diagram and tell us which part?
Do you mean buffer retaining spring? To keep the buffer and spring in the buffer tube? Sits under a buffer retainer (cap)? You’d have lost it while installing the buffer tube?... couldn’t be that I guess, it’s not a piston. Don’t be mad at us if you don’t know what the parts are called, they won’t know what you’re talking about over there either lol. If you want to learn let us know.
You also posted the original inquiry midday and midweek, not a lot of traffic at that time.
After all that, I still might send you the spare I have.