On the signed photo, she wrote, “Dear Ben, You found me, named me, and believed in me when no one else did. My thanks and love forever. Marilyn.”
When they were coming up with her stage name, the actor reportedly suggested Monroe because it was a name from her mom’s side of the family. The executive suggested Marilyn after his Her Majesty, Love costar Marilyn Miller.
Source: https://www.buzzfeed.com/kristenharris1/old-hollywood-stage-names-stories