I think they were implying it was from one of his court house signature stamps. The stamp had his assistants initials (D.A.) to differentiate it from his own in documents.
I took my stamp when I left my job that had one.
back in the day, when someone didn’t physically sign something but their secretary signed it, the sec would add their own initials indicating it wasn’t a direct signature. I often would sign for my boss and put my own initials there.
Maybe the signature was the secretary’s as well as her initials.