On my 1957 GMC 1/2 ton Napco, they are "stepped" wheel cylinders on the front (2 different sized cylinders). When I rebuilt them, I had to source the wheel cylinder rebuild kit from a Studebaker website (I don't recall which one).
Jim Carter Truck Parts has some for 55-57 Napco Trucks. Part # NA125L and NA125R. You can see them on their website for reference or call 800-842-1913.
