1/4 NPT with 1/16" Drilled Vent hole. HEX SOC PIPE PLUG PLATED.
This is used in our 18.75 to 1 and the 5 to 1 gear boxes as a Vent plug.
You only need one of the NPT plugs with the "Vent" hole to relief the pressure from building up in the gear case.
Link for the Standard NPT plug without the Vent hole.
http://www.staton-inc.com/store/products/1_4_NPT_HEX_SOC_PIPE_tapered_PLUG_PLATED-801-0.html