After saving Mark from a rattlesnake, the McCains befriend a stranger; but the McCain’s soon discover that he’s not who he says he is.
(From "The Rifleman Episode Guide")
One of those characters played by John Anderson merited further consideration.....
Eli Flack, as miserable a bastard as he was, still must have found a woman who loved him and bore him at least one son. But eventually she feared for the safety of her child, not to mention her own well-being, living with the violent prison guard. So she may have taken their son and headed East. (Perhaps she had family there.)
When Flack Junior grew up, he might have gone into law enforcement himself. In fact, it looks to have become a family tradition, leading to Detective First Grade Donald Flack, Jr. of the NYPD, who worked closely with the CSI unit in New York. (As seen in 'CSI:NY', of course!)