If you look at the sides of your seat you will find four spots for the pictured items. They are the bushings through which the grab handle bolts go through to hold your seat onto the bike. One of the spacers is what you have found.
Make sure when you install the bushing back into the seat that they are positioned correctly. The bushing is clocked so that it is only suppose to fit one way, but they tend to get mangled enough that they will fit anyway. I use Goop to fix mine into place.