DK was played by action star Charles Starrett in a series of literally dozens of films from 1940 until 1953.
Between his first film role in 1935 until his retirement, at the end of the DK series, Starrett appeared in well over 100+ movies. Depending on the various sites it ranged from 115 to over 160 films, mostly action/westerns during his career. I have to assume that some of the writers count his early non-starring roles, while others only those in which Starretts name appears in top of the credits. Either way it is an amazing run for any actor.
If you're at all interested you'll find more about Starrett and DK at IMDB.com, CowboyPals and AC Comics (where publisher Bill Black has issued reprints of some of the DK stories from Magazine Entertainment).