It originally had a tee on it. The lower end of the tee had a cap. When the airplane left the factory there was a sticker on the face of the cooler that told you to remove the cap and drain the cooler at oil changes.
It was mostly useless. The cooler sits sideways so that the ports are halfway up and only half the oil drains out, the clean half. The sludgy half stays where it was, in the bottom.
Cessna recommends removing the cooler and flushing it every 1000 hours. If that isn't done, the cooler gets full of baked-in crud and doesn't cool so well. You end up with high oil temps on a new engine. A new oil cooler fixes that.