won't be anything specific to 58/9 in the assy manual because it was printed in 55

but if you mean the one that's
similar to a door seal, see sect 11 sheet 5.02 viewA, the 'square' sides get glued to the back edge of the reinforcing piece on the back of the fender and mates to the hinge piller
there's also
a U-shaped piece that goes on the back edge of the inners where they meet the cab, and a smaller one that can be used where the back of the top edge of the fender is against the side of the cowl, or can be used at the front where the fenders bolt to the core support
Bill