I was thinking the same thing. But if there's supposed to be an o-ring there, where did it go? Not installed during manufacturing?
I'm not sure but orings do fail at times. The radiator tank is drilled and the cooler is installed before the tank is crimped to the radiator. It has to seal between the trans cooler and the tank somewhere. If it's on the inside, then the entire radiator is toast and needs to be replaced. I'm not certain it's an oring but there's definitely got to be a seal of some type somewhere.
If I was repairing it for the OP, I'd be replacing the radiator.