Were you looking in the NOS section? Of the two I see listed, one may be a complete assembly with bearing and one may be just the rubber support. You would need to call to see what the actual difference is because the description is not definitive.
I do not suggest buying an NOS rubber center support due to the unknown certainty of the age of the part. If its 20-40 years old, the rubber could be be dried out and may fail prematurely.
If you go to the the Standard Riviera Catalog, you will see the bearing itself can be purchased separately as well as rebuilt rubber supports. I suggest replacing both at the same time.
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Jason Zerbini
ROA/BCA