heat exchanger

Hello, I recently removed the heat exchanger core from my engine. There is a regular groove on one end. Should there be one? I don't see it in the diagram. Does it go on the front or the back? Thanks

I have a 1996 4jh te (serial #12285) and think the heat exchanger may be compromised. Here´s what happened. Started with a big mistake - we tried to fill the hot water heater heat exchanger lines at the hot water heater using a hose and a T fitting. (hot water heater is above the engine). We...