DR TCB that particular listing means that for THAT particular film, the actor known as Robert Walker, Jr was billed as Robert Walker.
I seem to remember that he did that for a couple of films to honor his father. It is true that SAG (and Equity) only allow one working actor with a name on its rolls.....but an inactive name can be re-used after an interval. I am not sure about the time frame, but I think in that particular case it was done for personal reasons......Harry Carey, Jr, used that name for his entire career, so it must have been RW Jr's strong feelings about his dad.