Hi JP,
I can't imagine a random threaded fitting being added to an ASME tank (what we have). I'm fairly familiar with our tanks (I'd bet dollars to doughnuts you have the same one I do), but I can't quite make out what angle I'm looking at in your photo.
Oh wait, I see you said it flipped your photo upside down. Then I think that's the relief valve mentioned in #5 below. You can see on my photo of "our" tank that corresponds to the hole in the shield that also shows in my photo.
Here is a photo of "our" tank for starters, looking at it straight on.
From left to right here is what I understand the various things to be:
1) Sight gauge (has needle)
2) Fill (should have yellow plastic cap, although it may be missing since it's just plastic)
3) Fixed liquid level gauge (little pinhole in a square looking thing surrounded by a reddish circle) (also used when filling)
4) On/Off valve (may have more official name, but this is how you turn on and off the propane to the regulator and thence the interior appliances).
5) Relief valve (you can barely see it behind the regulator but it points down). This is what will release the propane in case of a road accident or etc. - this is why a tank may shoot (relatively harmless) fire in an accident but doesn't "blow up."
6) Regulator (the two metal "circles" on the right between the on/off valve and the black hose)
It's possible to add things (such as an extend-a-stay or a way to get unregulated propane for a barbecue or something) but these things are added "in line" and do not pierce the tank itself, as far as I know.
Here is a link to an article that show how some of the "guts" look on the inside and what they do:
http://roadtreking.com/innards-rv-propane-tank/
As I remember it their tank is laid out slightly differently than ours, but the same principles apply.