Okay - The 1929-41 Chevy Master Parts Book does not list a filler plug of any sort for the 1941 truck steering box. Next best/closest things is what's listed as a "Bolt, steering gear housing breather" for the 1938-40 truck boxes.
Perhaps the hole in the housing was GM's initial thought for venting the gear box differently for 1941, but later replaced by the vented filler plug in 1942???
