On my 52 GMC there were rectangular holes in the inner fenders not the the toe board of the cab. These holes on the inner fenders are for rectangular captured nuts. There are 5/16 x 18 cap screws about 3/4 to 1 inch long that secure the inner fenders to the toe board screwed in from the cab side. Several of mine were missing too so I made replacements and welded them to the inner fenders.