I have seen multiple cards with this particular designation on Ebay before. I am not sure of the exact details (though I'm sure a little research on Google could yield the answer), however I have seen auction houses take "raw" (ungraded) cards and send them in for grading on behalf of a consignor. If, for example, an auction house (say Robert Edwards, or Heritage Auctions, hypothetically speaking) were selling a collection of cards from the Mantle estate, they would have the provenance necessary to prove ownership.
It is not uncommon to see this additional "byline", if you will, added to a flip. PSA, and I'm sure SGC and Beckett, offer this service for an additional card. One such example of this that stands out in my mind is the Dmitri Young collection (cards graded by PSA). There are countless others.
I hope this sheds a little light on your question. Good luck!