This is the thing in question
Here is a bigger picture. Main natural gas line comes in, this thing is attached before the meter. A half inch natural gas line runs out of it towards the front of the house, where there used to be a gaslamp (still there, now electric).
I think I remember the gaslamps burned all the time because resources are forever, or at least they used to be. So my guess is pressure regulator or emergency cut off, or both???
House was built in the 50s if it matters, though the meter may be a little newer.