Serial number was stamped around mid-June 1920.
One interesting feature of this guitar is the coarse grain structure of the wood used for the neck. This type of wood is Philippine Mahogany (genus Shorea) which is not considered a true mahogany (genus Swietenia). It was used for the necks of Martin guitars circa 1919 to 1920.
Style 18 features:
Back and sides: Mahogany (probably Mexican)
Top: Red spruce (Adirondack)
Backstripe: Single line of rosewood purfling
Other period features:
Ebony fingerboard (original?)
Rosewood pyramid bridge (original?)
C. F. Martin & Co. stamp on back of headstock
No style stamp
At the time of writing this guitar is available at Schoenberg Guitars (http://www.om28.com/ProductDetail?product=P150603001)
Photos courtesy of Schoenberg Guitars