Vance & Hines, 12mm O2 sensor plug set. Zinc.

Zinc plated steel. Fits 12mm x 1.25 sensor bungs. Required with non-O2 (oxygen) sensor equipped models that have an exhaust system installed with 12mm threaded sensor bungs. Used when O2 sensors are no longer required.

Fitment:

09-25 H-D with stock 12mm sensor ports