Cutaway drawing on Service Sheet 212E indicates this cover 42 6328, listed as "Cap", is pressed into the empty hub from the chainwheel end, seating on a ledge halfway down the bore. It acts a grease retainer of sorts, and also serves to centralise the bearing distance piece 42 6323 as the main spindle is inserted from the other end of the hub. As such it rotates with the wheel hub, and is a sliding/loose fit on the distance piece.
Swarfy.