According to Bob Brozman, The History and Artistry of National Resonator Instruments, page 85, the Style O is nickel plated over a bell brass body, has a single cone, and features sand-blasted Hawaiian scenes on the body. It has a maple neck in a "black to clear" sunburst finish on the back. The fingerboard is maple but stained black (one can see the light colored wood under the black in some positions). It has mother of pearl dot markers - this one has 7 markers in 6 positions with a double dot at the 17th fret. National Style O guitars have flat-cut f-holes in the period 1930-1933, as has this, and then rolled f-holes from 1933 to 1941. You probably also want to know that from 1930 to 1934 the guitars were twelve-frets to the body and then from '35 to '51 they were 14-frets to the body.
In this period the model originally came with a National headstock decal but this delightful Dalmatian has lost its decal. Nevertheless it looks regal and stately with a dyed black overlain headplate with long oblong slots, having its original rectangular plate brass colored open-gear tuners with grained ivoroid buttons. The nut is bone; the fingerboard is bound in crème ivoroid on three sides. This guitar has the standard cover plate pattern of 10 diamond shapes each made up of 41 small holes. The sides are sand etched with shiny finish on each side and dull at the larger center swath. The guitar shows normal light dings and dents, finish checking and finish wear in places but surprisingly little on the back of the neck and back of headstock; the hand rest over the biscuit bridge is less worn than one normally sees, and so is the face and the cover plate. The black biscuit bridge is original and impressed (to the best of our ability to discern it) "Pat 1806756, Other Pat. Pend." The serial number is stamped into the bottom side near the end pin. The trapeze tailpiece is original. This guitar has a small rise (just like they say when the judge walks in) in the fingerboard around the 12th fret but this is right now not affecting playability. Its finish is normally (marginally) cloudy the way old nickel plating gets. On the whole this is actually an amazingly clean and superb sounding resophonic instrument, fit for another hundred years of playing, but only if you have a large wrap-around porch on which to sit, in the shade, and you are, as well, wearing a Panama hat.