What's this 'plug plate'?

Hi,

I have a petrol 2RZ-E engine and I'm going to install a watchdog. I've got a two source device so I'm going to try a few different locations to ensure I pick one that's looking accurate.

I have a provisional location that's next to the water outlet but I've no idea what it's for. There are two bolts holding a 'plug plate' next to the water outlet. It covers a hole in the head that makes the timing chain visible. It seems a good place to try but I was curious as to what it's for?

I don't think that does anything, even as a way of looking at the timing chain it's a bit daft when you can take the rocker cover off.

My bet is that it's for some component that your van doesn't have so they could use the same mold for a cylinder head. Maybe someone can tell you what they've got there to confirm my suspicions.

Going by the diagram it looks possibly like an access to the chain tensioner. ??

No wait, is the tensioner the one below it?