Thanks to all who submitted ideas on what the stamp in area 11 of the SS5 card meant.
I found several webpages on the Social Security Administration website that gave me some perspective--and more information that I really thought I would ever need.
Operations in the Candler Building 1936-1960
HISTORICAL INTERVIEW WITH JACK S. FUTTERMAN
Oral History Collection
Robert P. Bynum
Based upon this reading (and the submissions of several members of the APG list, the Gen-Newbie List, and the Roots-L list), I am going with
which I am inferring to mean that the Division of Accounting Operations at the Social Security Office had no record of her employment. This would explain why the stamp was in the region of the card that it was. Anna Lake had worked before 1963, but that work was before her marriage which would have been before 1930--which was before Social Security anyway.
Thanks for all the suggestions about the card.
Next question: what made her get her card in 1963? See how one thing always leads to another?