The Martin 0-28K was made between 1917 and 1935. It was an all koa steel stringed version of the Martin 0-28. The koa tops were used in part because of a belief at the time that spruce soundboards could not withstand the force of steel strings. Makers like Martin noted the use of koa & steel strings on Hawaiian made instruments and followed suit. Total production of the model was 641 were made in total. The 0-28K, at least until 1929, had the bridge set up in such a way that it could be easily converted between Hawaiian and Spanish configurations by simply raising the nut.

Source: vintage guitar 0-28K article