Stainless steel M18 bolt and washer to cap off an unused Oxygen Sensor (O2 sensor) that use the round M18 connection. This is the same as all the common wideband controllers and O2 sensors used with stand-alone ECUS.
Many older Toyotas, including the AW11 and SW20 also used the flange-style O2 sensor that used two M8 nuts. This plug will not work for those without an adapter first. .
The fastener style may vary slightly from the photo.