Originally Posted by NorthwestBBQ
They are electrical Knockout Filler Plugs-Caps:
I went to the hardware store to pick up some of these today. They had half and 3/4" variations, so I grabbed a few of the half inch caps... they are a little too large to fit in the lower air vent holes on the WSM 18.5". What size did you buy and do you know if they are available in a size smaller than half inch?