This is a floor pan grommet which will seal various parts along the floor pan and inside the spare tire compartment.
We measured the larger diameter of this plug to be approximately 53mm, and the smaller diameter to be approximately 46.
Manufacturer note: Size and quantity of floor pan plugs used on 240Z, 260Z, and 280Z varies from car to car. We recommend measuring the size of the holes in the floor pan to make sure this is the correct plug for your vehicle. This grommet fits a 44mm hole.