Size Frets Width Depth Length Introduced ---- ----- ----- ----- ------ ---------- O 12 13 1/2 4 3/16 19 1/8 1852 O 14 13 1/2 4 1/4 18 3/8 1934 (first year for 14 frets) OO 12 14 1/8 4 1/16 19 5/8 1877 OO 14 14 5/16 4 1/8 18 7/8 1934 (first year for 14 frets) OOO 12 15 4 1/16 20 7/16 1902 OOO 14 15 4 1/8 19 3/8 1934 (first year for 14 frets) OM 14 15 4 1/8 19 3/8 1929 (discontinued 1934, reintroduced 1977) D 12 15 5/8 4 3/4 20 15/16 1931 (available 1916 from Ditson) D 14 15 5/8 4 7/8 20 1934
The OM model is basically a 14-fret 000 with long 25.4" scale. The 14-fret 000 has a 24.9" scale.
(Thanks to Felix Wiedler.)