Just found this forum post because I was looking for the exact same answer! I'm assuming you've figured this out by now but in case you haven't... This is a piece of the insert that goes into the plunger for the under playfield kicker/loop. You can purchase just the insert and try to get it back into the existing plunger but I doubt you get that to work. Check out part number A-17767 which is the entire plunger and insert assembly.
Thanks!
DLO