Visible crew/equipment: After Captain Stanley tells the guys about the doctor who's doing the diet/nutrition study, when they leave to check the new refinery hydrants, the actor's T-mark is visible on the floor where DeSoto had been standing. Also note that DeSoto is wearing slippers when he stands up. It happens again later, he's wearing slippers with no socks when Morton is talking to the guys about stress and diet.
Visible crew/equipment: When the guys are looking at the vitamins that Chet brought for them 51's tones drop, and as they leave the kitchen the two actors' T-marks are visible on the floor where Mike and Marco had been standing.
Answer: I believe you are speaking of the old man, Mr. Wilson. He's played by J. Pat O'Malley.
Bishop73
Correct, it was J. Pat O'Malley...he also played the grandfather with his grandson when their rocket exploded, and also played "Old Bill" in the episode with Ann Prentiss, where Gage saves the little girl from the burning tree house, and her mother falls in love with him.