I AM GOING TO SAY THAT THESE TENNIS BALLS ARE RARE & UNIQUE judging by my research and the can's aging.  The canister is made of CS metal and not plastic. A sticker on top of the cover states the color to be "Optic Yellow." THE CANISTER IS STILL SEALED, Fun collector's item! Vintage MacGregor Optic Yellow Wool Nylon Heavy Duty Tennis Ball 3 Ball Canister. #171-015 U55