Drop front/field desk

My father bought this desk, the table it sits on, and two glass-front bookcases in 1968.

Also included were papers dealing with Gen. George Stoneman in his post Civil War Years.

These papers were donated by my Father to the Huntington Library.

The desk is 27 1/2 inches tall, 36 inches wide, and 13 inches deep.  It appears to be made of Walnut, and is put together by square nails.

I'd like to know if this could be a field desk, or is this a design that was commonly found in households and commercially in it's time.

Thanks for your consideration,

Allen Carter