A question a while back on a historical writer's loop was searching for when clipboards were in use during the 19th century. Thanks to Carla for her links to the email loop with the answers that gave me further direction in answering this question.
Here are Carla's references:
An Attorney General's report 1880 lists the item.
The Writer Vol. 1-2 referencing a letter-clip with a description of the board.
I found some earlier references:
Below is an image of a letter clip in 1842 from The Practical Mechanice & Engineer's Magazine Vol. 1 Page 32.
Referenced in the Household documents of an estate.
Referenced in a Patent book as similar to a letter-clip.
A Practical Dictionary with a description of the item.
The New Letter-Clip