The M-210 message book, part number
6D2107-210, is an M-209 accessory. It is listed in TM 11-380,
dated 17 March 1944. This book is used to record messages from
any source and save a carbon copy of the message in the book
when the original is removed and given to the intended
recipient. At the back of the book, there is some extremely thin
paper as well as regular message paper. It is believed that the
thin paper was used with carrier pigeons. Yes, carrier pigeons
were used by the US Army to transport messages.