“Leonard” is not a member of the cast.
You guys are on the right track, but haven’t found the right answer yet. Sheldon is obviously her true soul mate, not Leonard. That means Penny’s last name will be Mini. So when they marry, she becomes … Penny Mini-Cooper.
That article is from 2009. They had only two seasons of the show at that point, and it was pre-Amy and pre-Bernadette. I don’t think there’s any time limit set on if and when Penny’s last name gets revealed.
What Nars said.
Among your “100 other examples” do you have one in which the cast would have to have that knowledge?