Should there be a O-ring on the mounting surface of the timing chain tensioner

Yes, that would be a oil seal to keep the tensioner lubricated.

That seal is below the mating surface. Should the orifice be sealed against the head?

Is there a fitting that protrudes out from the block? And is that seal in a counter bore. I haven't replaced one of those, but the seal is there for a reason. What does the old one look like?

No fitting protruding and have discovered that the recess in the tensioner is slightly embossed but below the surface.. surely it is not a metal to alloy head mating? I am inclined to believe that there should be some sort of rubber seal here

I found where I could get a close up view of the tensioner. There is a screen behind that "O" ring and it looks like the o-ring actually sits below the mounting surface. I checked several sights and all the tensioners look the same with that o-ring. Its probably fine the way it is. Just my opinion. May want to check with a Nissan Forum to see what they say, I've read threw several but I have no desire to join one.

Did you take the old one off to see how it is, how it looks, is it the same way?

Took the old one off but it is missing the screen. I am not sure if there was a seal. It could have fallen off during disassembly

The counter bore looks the same depth? I would think oil flows into that hole from the engine.

Yes that is true but my issue is whether there should be a seal against the mating surface on the head

In looking the part up, I don't see any gasket listed to go on. When taking the old tensioner off, you would have seen a gasket there, if there is one suppose be there.

Thank you so much for your help. I apparently does not have any seal of any type and is just a face to face fit although I do note that the tensioner appears to be coated in a graphite type finish. I could not get anyone from Nissan service to speak to me so I checked with spare parts and was told that there is no seal and my conclusion is correct. So to be safe I will spray a coating of Hylomar on it before installing.