Don't know where that oddly-placed speck came from-- digital blank or analog blot (I didn't make it, or even intentionally pause the machine at the right place)-- but it seems to be the episode where Raymond Burr was accidentally shot in the back of the head by a clumsy cast member.
The show went on hiatus for retooling-- there was some talk of "Hamilton Burger, District Attorney" coming to the airwaves-- but a little bullet through the noggin wasn't about to stop Raymond Burr, and he returned unharmed the next fall. The summer replacement was "Agent 13, Man of Mystery", starring Dave Ketchum.
I do have some odd Mason screen caps around here somewhere...