Karl turned out to be much younger, probably born in 2002, like Curtis. Despite being on the street in the middle of winter, he's a calmer cat than Curtis. They tussle and gives as good as he gets.
Karl is distinguished by having seven toes on both front paws (polydactyly). He is very cuddlesome, sometimes demanding cuddles at odd hours, like 4 a.m. He has visited very nicely with our older cats, who rudely hissed at him. We'll be baby-sitting him in the near future.
Jennifer hasn't totally bonded with Karl yet, though he has with her. She says, "You could sort of say that Curtis is my cat. Karl is Curtis's cat." Goofy...