In this particular guitar, the "K" tells us that the back and sides are made of highly prized Hawaiian koa wood.
This one has hardly been used--if it were not for some tiny rubs on the pickguard, it would pass as new--no scratches or dings, and zero fret wear! The original case is equally like new.
With that original hardshell case it's only $3500.
Martin's fancy "40-series" guitars are valued mostly for their inlay, specifically, the abalone binding. In function though, they are HD-28s dressed up to varying degrees. The D-42 is the exception, though, as Martin uses their forward shifted, scalloped bracing in this model, giving it a sound much more like their Vintage series guitars.