The MGA With An Attitude
REAR HUB Dust Cover - TC-209A
BMC part number ATB7269

This part should not be a problem, except that we re a bit short on information. It is item number 43 in the illustration at right (from the Service Parts List). On the pushrod cars with rear drum brakes, there is a labyrinth arrangement in this area that may help keep dust out, but is surely intended to direct any oil leaking from the hub to run down the inboard side of the brake backing plate (to avoid getting oil on the brake shoes).

I find cross section illustrations of the Twin Cam (and "Deluxe") rear axle hubs in the Workshop Manuals. This "Dust Cover" appears to be a labyrinth ring (with similar purpose) that may be press fit into the caliper mounting adapter plate.

Would there be an oil drain groove on the inboard side of the caliper mounting plate (or in the axle housing flange)?

Here is a picture of the bearing hub and the adapter plate, which appears to have the dust cover installed (viewing the inboard side of the plate). The problem is, a lot of cars seem to be missing this part, lots of people don't realize it is supposed to exist, parts suppliers do not have this part, and apparently no one notices any problem if it is missing. If it is indeed a drip redirector, then I would recommend that you try to include it in the assembly.
Here is another picture of the Twin Cam rear caliper adapter plate with the Dust Cover press fit in the large center hole (again looking at the inboard side of the plate).

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