[Rick Parker]

The center hole registers the drum on the axle flange, the studs often are not "exactly" perpendicular to the axle flange surface and may not provide enough support to keep the drum concentric. If there is enough metal in the adapters, (measure the thickness of the material of the major diameter at the flange) you may be able to make a shallow 45 degree chamfer on the backside inside diameter of the adapter that will allow the adapter to be fully seated against the drum.
Mark the adapter with a black felt pen or blue machinist dye, install it with the nut and then remove and look for the "witness markings" on the adapter. From your photos it appears that the adapter "Inside Diameter" is clamping d marking very close to the OD of the raised diameter on the drum, you may only have to remove small amount of material from the adapters.

Rick