20/1/67 one episode ‘The Night of the Gypsy Peril’. Mark played a young cowboy called Hillard. Also starring Robert Conrad and Ross Martin. In parts of this show, Mark Slade’s voice has been dubbed with that of another actor, as when he was called to do a ‘looping session’, Mark was away on another location and was not available to dub it himself.
The episode revolves around an extortion plot involving the theft of a baby elephant (which Hillard/Mark loses!)