Sorcha,
Here is the info from George Gruhn's "Guide to Vintage Guitars":
Style 3: mahogany body, 7-ply top binding, 3-ply back binding, 5-ply soundhole ring, celluloid (or bone) ornament lower edge of top, 17-fret ebony fingerboard extends to soundhole, small pearl paired-diamond inlay at frets 5, 7, and 9, diamonds joined at 7th fret, 3 lines inlaid down center of fingerboard, nut of 3-ply plastic, 4-point celluloid (or bone) peghead ornament, friction pegs
Introduced: 1918
Koa wood 3K available: 1920
Unjoined diamond inlay at 7th fret, no peghead ornament, nickle patent pegs: 1923
Style 3 last listed: 1939
No celluloid ornament at lower edge of top: late 1940s
Style 3 still in production (koa available)
This book is copyrighted 1991.
One other point. The "C.F. Martin" stamp on the back of the headstock dates it to pre-1935.
Hope this helps...!!!