Serial number 50306 is a 1932 dark top 00-40H. It was stamped on February 29, 1932 and cleared final inspection on June 14 that year.
At least two 00-40H guitars were made with dark tops around this period but there may be more since Martin did not always record this feature, especially if a customer made a last minute change to an order.
One interesting feature is the extra cat's-eye inlay at the 17th fret which is usually only seen on style 45 guitars.
Like many Martin Hawaiian guitars this instrument was converted for Spanish playing by replacing the original flat fingerboard and flush frets with a rounded fingerboard with raised bar frets. The raised nut and saddle were also replaced.
The tuners are clipped plate Grover G-97 machines with seamed butterbean buttons.
Style 40 features:
Back and sides: Brazilian rosewood
Top: Red spruce (Adirondack)
Backstripe: Style 45
Other period features:
Ebony belly bridge
Ebony bar neck reinforcement
1-7/8" neck width at nut
Tortoiseshell celluloid pick guard
Style 40 pearl fingerboards inlays with extra "cat's-eye" inlay at the 17th fret
C. F. Martin stamp on back of head stock