The rub rail mounting piece is not aluminum, but a plastic extrusion (maybe pvc?). It is also not a 'U' channel but more of an 'I' that is curved, one set of legs pressed against the hull and covering the joint between hull and deck, with the other set of tabs serving to seat the vinyl/rubber rub rail, almost like a dovetail.
I could probably employ and new vinyl/rubber rail and redo the entire length of the boat, serving the same protection of the joint and still looking respectable, but this would run more $$ and take more labor than replacing the 6 ft or so of hard plastic molding that got cracked.