Rusty Stevens appeared on an episode of The Rifleman that Me-TV reran today, and it was originally aired during that show's Season 5, which was the same year (1962-63) as LItB Season 6, so he was still working even after leaving LItB. It was called "Hostages to Fortune", and he played a boy named Melvin who was picking on a British boy named Percy who'd just moved to the US with his dad. Melvin started shoving Percy around when Mark interfered and got into the fight with Melvin, leaving both boys exhausted.

Kinda busts the theory that he moved to New Jersey, since he also had bit parts in a few other shows around that time, per IMDb.

I know that episode and he’s quite a bully in that one. Love how Percy calls him “fat boy.” Yup surely knocks down the theory he moved away. I bet the reason he left LITB was because of his mother like Barbara tells it.