*To me* it looks like a T&P relief valve, if so, it was probably one of the ones with a short probe, which is why it is installed into a tee like that (the probe is down in the tee).
The next question, then, becomes: Is it acceptable to install and T&P valve were only a pressure relief valve is needed?
I would say 'Yes, provided that the T&P pressure setting is at the same pressure setting as the pressure relief valve would be. "Provided that" is the key, like saying *IF*.
