T&B Steel City BL 1″, 1 1/4″, 1 1/2″, 2″ Snap In Conduit Knockout Filler Blank Steel City® knockout blanks are used to close off knockouts in boxes or panelboards. Knockout blanks are constructed of zinc plated steel. 1", 1 1/4", 1 1/2", 2" Snap In Conduit Knockout Filler Blank. byJuly 4, 2026