This nylon ring is above the large ball and below the lower shift boot on your shifter, it also holds a wave washer when performing a shifter rebuild.
Suits MX5 NB 6 Speed with large 21.6mm lower shift bush on shifter
If you are rebuilding the shifter choose the one for your model below
NA 5spd shifter CLICK HERE
NB 5spd/small bush 6spd shifter CLICK HERE
NB large bush 6spd shifter CLICK HERE