That's funny that you should put a picture of a 5/8 drive socket set. I just picked up a modern Proto 1-1/4 inch six point socket in 5/8 drive. It is a black impact socket # 7640-H. A odd ball for sure.
Someone over at the Garage Journal said that CP made a impact tool in that size.
EvilDr235 / Gort the giant robot.